Aims: To study the chemical composition of volatile oil samples from different place in Egypt (Mansoura, Gamsa and Assuit) and study their antioxidant activity determined by DPPH assay. Study Design: Hydro distillation of volatile oil samples and their GC/MS analysis and determination of their antioxidant activity by DPPH assay. Place and Duration of Study: Department of Pharmacognosy, Faculty of pharmacy, Mansoura university, Egypt, between June 2015 and November 2017. Methodology: The

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... l oil was isolated by hydro-distillation for 5 h using a Clevenger-type all-glass apparatus according to the standard procedure of the European pharmacopeia and volatile oils analysis was performed by GC and GC-MS. GC analysis was carried out using Focus-DSQ-ӀI GC/MS instrument (Thermo Scientific, MA, USA) equipped with TR-5 fused silica column (30 m × 0.25 mm, film thickness 0.25 µm). Results: The yield of the essential oils of three rosemary plants growing in Mansoura (RM), Gamsa (RG) and Assiut (RA) were 0.20% v/w, 0.32% v/w and 0.24% v/w, respectively. Their chemical composition was analyzed by GC/MS, RM and RG were found to be α-pinene dominated chemotypes, 32.4% and 29.6%, respectively, meanwhile RA was camphor dominated chemotype (17.2%). Furthermore, their antioxidant activity was determined by DPPH assay. Their IC50 values of essential oils of RM, RG and RA were 8.66 ± 0.7, 8.18 ± 0.5 and 9.74 ± 0.2, respectively. Conclusion: The GC/MS spectral data revealed the considerable difference between the chemical composition of essential oil constituents of RM, RG and RA which lead to different chemotypes. The present results also demonstrate that REO obtained from different areas in Egypt exhibited free radical scavenging activity determined by DPPH assay due to the synergistic effect between their constituents.