At the last decade, biodiesel has appeared as one of the most potential energy to replace the current petro-diesel. It is biodegradable, non-toxic and renewable fuel which can be easily fabricated through esterification of free fatty acid or esterification of triglycerides (animal fats and vegetable oils) with methanol. The main objective of the present work is to synthesis a magnetically recyclable solid catalyst for the esterification of waste date seed oil to meet the standard acid value. The idea of using waste Date seeds for biofuels production is promising due to its widely available in Oman. The oil from the waste date seeds was extracted and characterized to use as a biofuel. To accomplish this, Fe3O4 nanoparticles was fabricated by using solvothermal synthesis method, and then the mercaptoacetic acid was physically coated on it. Different techniques such as XRD, FT-IR, SEM/EDS, BET, VSM, Water-Toluene test and nitrogen adsorption-desorption analysis were used to characterize the as-prepared solid catalyst acid. The characterization results indicated that Fe3O4 nanoparticles had been successfully coated with mercaptoacetic acid. Furthermore, the solid acid catalysts possessed superparamagnetic behavior and high saturation magnetization, allowing them to be facilely separated from the reaction mixture by using an external magnetic filed. The esterification reaction was modeled and optimized by RSM (Design Expert program). The solid acid catalyst seemed to be an efficient and environmentally benign catalyst for the esterification of date seed oil with methanol for the free fatty acid conversion, giving 90 % FFAs conversion to methyl esters within the optimum conditions of time contact, catalyst loading and temperature which about 47 min, 1.5wt %, and 55 o C respectively. The Methanol: Oil ratio was used 15:1 as a constant independent variable. The solid catalyst could be readily recovered by simple magnetic decantation and reused for several times without significant degradation in its catalytic activity.