Four series of nanoparticle perovskites (La1-xPrx)0.7Sr0.3MnO3 (LPSMO) (0.0 ≤ x ≤ 1.0), (La1-xTmx)0.7Sr0.3MnO3 (LTSMO) (0.0 ≤ x ≤ 0.18), La0.7Sr0.3Mn1-xFexO3 (LSMFO) (0.0 ≤ x ≤ 0.30) and Pr0.64Sr0.36Mn1-xFexO3 (PSMFO) (0.0 ≤ x ≤ 0.30) were synthesized at low temperature (700 - 800 o C) by th e sol-gel method followed by sintering. All samples were studied and characterized using X-ray powder diffractometry (XRD), transmission electron microscope (TEM), Field-emission scanning electron microscope equipped with an energy dispersive spectrometer (FESEM/EDXS), Fourier transform Infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometer (VSM) and Mössbaour spectroscopy. XRD analysis shows that all samples are single-phase perovskites with no impurities detected. LTSMO and LSMFO perovskites crystallized in the Rhombohedral structure with the R-3c space group, whereas PSMFO perovskites crystallized in the Orthorhombic structure with the Pnma space group. On the other hand, LPSMO perovskite has a rhombohedral structure (R-3c space group) for x ≤ 0.4 and an orthorhombic structure (Pbnm space group) for x ≥ 0.6. All perovskites are in the nanoscale size with an average particle size in the range of 21- 70 nm. The FT-IR spectra for all samples show the appearance of two bands ranging between 418 cm-1 and 634 cm-1 , which are attributed to M-O (M=La, Pr, Fe, Sr, and Mn) stretching vibrations and M-O-M Bending vibrations. The appearance of stretching and bending vibration modes indicates the formation of the perovskite structure in agreement with XRD results. According to the XPS results, La, Pr, Tm, and Fe oxidation states are +3, Sr +2, and O -2, whereas Mn has a mixed valence state of Mn4+/Mn3+ and the ratio of Mn4+/Mn3+ remains almost the same. All samples exhibit a ferromagnetic (FM) to paramagnetic (PM) phase transition at Curie temperature (TC). The TC for LPSMO, LSMFO, and LTSMO nanocrystalline perovskites decreases linearly (starting from 370 o C) as x increases. However, for PSMFO nanocrystalline perovskite, the TC decreases linearly for low x concentrations, while the decrease in TC is almost small for x ≥ 0.16. Second-order phase transitions are observed in the LPSMO, LSMFO, and PSMFO nanocrystalline perovskites, and the magnetic coercivity (HC) for these nanocrystalline at T close to TC was found to be small (~ 10 Oe). The irreversibility temperature (Tirr) was observed to decrease for all investigated samples. As x increases, the saturation magnetization (MS) of LSMFO, LTSMO, and PSMFO perovskites decreases. For LPSMO perovskite, the MS for samples with an orthorhombic structure increases with x, reaching a maximum value of 94.0 emu/g at 5 K for x = 1.0. However, for those with a rhombohedral structure, the maximum MS (87.2 emu/g at 5 K) is achieved at x = 0.2. The maximum value of the magnitude of the magnetic entropy change (-∆SM)max and the relative cooling power (RCP) for orthorhombic LPSMO perovskite increases with x content, reaching a maximum value of 2.92 J/kg.K and 350 J/kg respectively, at ∆µoH = 5 T for x = 1.0. On the other hand, the rhombohedral LPSMO has the largest (-∆SM)max and RCP of 2.96 J/kg.K and 271 J/kg at ∆µoH = 5 T for x = 0.2. For PSMFO nanocrystalline perovskite, as the Fe concentration increases, the (-∆SM)max and RCP decrease for all the samples, and Pr0.64Sr0.36MnO3 has the largest (-∆SM)max and RCP of 3.16 J/kg.K, respectively, and 307 J/kg at ∆µoH = 5 T. For LSMFO perovskite, as the Fe concentration increases, the (-∆SM)max decreases, while RCP increases for x≤0.08 and then decreases for x ≥ 0.12. Lao.7Sr0.3Mn0.92Fe0.08O3 sample has the largest RCP (153 J/kg) at a low applied magnetic field (2.8 T) and a phase transition temperature near room temperature. The Room temperature Mössbauer spectra for LSMFO and PSMFO perovskites consist of a single quadrupole doublet. The quadrupole splitting (∆) of the fitted doublet in the Mössbauer spectra at 300 K increases linearly with increasing Fe concentration. However, the range of variation in (∆) for PSMFO (0.31 - 0.50 mm/s) is greater than that of LSMFO (0.28 - 0.39 mm/s). The isomer shift values (δ) for LSMFO and PSMFO perovskites range from (0.33 - 0.37 mm/s) and (0.31 - 0.37 mm/s), respectively, indicating the presence of a high spin Fe3+ state. This study demonstrates that substituting Mn3+ with Fe3+ at the B-site of LSMFO and PSMFO, and substituting La3+ with Pr3+ or Tm3+ at the A-site of (La1-xREx)0.7Sr0.3MnO3 can alter the phase transition temperature TC and significantly change the magnetic properties of the samples. In the case of LPSMO manganites, samples with 0.4 ≤ x ≤ 0.8 are considered promising candidates for room-temperature magnetic refrigeration applications. The parent compound (x = 0.0) of PSMFO manganites has the best (-∆SM)max and RCP values, as well as a phase transition temperature that is close to room temperature, making it suitable for room-temperature magnetic refrigeration applications. The LSMFO manganite sample with x = 0.08 is regarded as the best candidate for applications requiring refrigeration at room temperature. Among all three systems, LPSM manganites may be considered good refrigerant materials at room temperature or in the vicinity of room temperature.