Frankincense is one of the cultural symbols of Oman. Throughout history, it has been used to enhance the wellbeing of individuals, in medicine and in perfumes. Omani frankincense is produced by B. sacra trees and is graded into Hojari, Nejdi, Shazri and Sha'bi; the first is considered as the highest quality and the last the lowest. This study investigated the physicochemical properties of the oleo-gum resin of the Hojari and the Sha'bi frankincense, and the chemistry and the antimicrobial activity of steam-distilled essential oils of these two grades. Chemical analysis was performed using gas chromatography mass spectrometry (GC-MS). The antimicrobial activity of the oils against Staphylococcus aureus (NCTC 6571), Bacillus spp., Escherichia coli (NCTC 10418), Pseudomonas aeruginosa (NCTC 10662), Saccharomyces cerevisiae, Candida albicans, Aspergillus flavus, A. ochraceus, A. niger, Penicilliuin citrinum, Alternaria alternata and Fusarium solani was screened using a well diffusion method. Minimum inhibitory concentrations (MIC) and minimum bactericidal/fungicidal (MBC/MFC) concentrations of the oils were determined using a mico-well dilution method. MICs were determined both visually and spectrophotometrically. No significant difference was found between the length, pH and hardness of the oleo-gum resin of the Hojari and the Sha'bi grades while the weight, solubility, moisture content, Young's Modulus, and color parameters (L*, a*, b*, chroma, and color index) were significantly different. Hojari and Sha'bi oils were similar in composition and contained monoterpenes and sesquiterpenes but not diterpenes. Terpinolene was the major compound and its level exceeded 50% of the oils of both grades. Other major monoterpenes were camphene, (+)-3-carene and 1-limonene. The Hojari and the Sha'bi oils possessed broad spectrum antimicrobial activity. The largest growth inhibition zones were obtained with S. cerevisiae and F. solani. An MIC of 1.56% (v/v) was found with E. coli, S. cerevisiae and F. solani. Utilizing the micro-well dilution method, the spores of moulds (except F. solani) were more resistant than bacterial and yeasts cells indicated by their high MFCs of 50% (v/v) or more. It was concluded that the physical properties of the oleo-gum resin of Hojari and Sha'bi frankincense have potential use to standardize the grading system in Oman. Hojari and the Sha'bi frankincense oils had similar chemical composition and antimicrobial activity