Objective: To analyze the main constituents of yoltile oil from Simo Decoction. Methods: Volatile oil was obtained by steam distillation technique. The chemical constituents were separated and indentified by GC/MS. Results: There were 67 peaks shown from the volatile oil, from which, 29 chemical constituents were identified, which accounting for 61.27%of the volatile oil. The main constituents in volatile oil are Aristolene(9.86%), 2, 4-Diisopropenyl-1-methyl-1-vinylcyclohexane (6.29%), myristic acid(5.92%), 3-ethyl-3- hydroxy androsterone alkyl-17-ketone(5.9%) and cetylic acid(4.17%). Conclus...