OBJECTIVE：To study the compositi on of the volatile oil from Compound chaihu guizhi decoction ，and to evaluate its in vitro anti-proliferative activity on human lung adenocarcinoma A 549 cells. METHODS ：The volatile oil from Chaihu guizhi decoction was extracted according to the steam distillation method of general rules 2004 in the 2015 edition of Chinese Pharmacopoeia（part Ⅳ）. The volatile oil components were analyzed by GC-MS combined with Kováts index ，and the relative content of each component was calculated by peak area normalization method. Using different concentrations of cisplatin （4，8， 16，32，64 mg/L）as positive control ，MTT assay was used to detect the inhibitory effects of different concentrations of volatile oil from Chaihu guizhi decoction （25，50，100，200，400 mg/L）on in vitro proliferation of A 549 cell after 48 h of treatment. Negative control group （with cells but without drugs ）was set up. RESULTS ：A total of 71 chemical components were isolated from the volatile oil ，among which there were 59 compounds identified ，sum of peak areas accounting for 84.99％ of the total peak area. The compounds with relatively high content included ar-curcumene （17.65％），β-bisabolene（9.57％），β-ocimene（7.05％）， α-curcumene（5.35％），2，5-dimethylbenzaldehyde（4.24％），linalyl isobutyrate （2.70％），α-cedrene（2.48％），δ-cadinene （2.07％）. Compared with negative control group ，the proliferation rate of cells were decreased significantly in 4-64 mg/L cisplatin groups and 25-400 mg/L volatile oil from Chaihu guizhi decoction groups （P＜0.05）. IC 50 of cisplatin and volatile oil from Chaihu guizhi decoction to in vitro proliferation of A 549 cells were 10.150 and 73.526 mg/L. CONCLUSIONS ：The volatile oil from Chaihu guizhi decoction mainly includes ar-curcumene ，β-bisabolene，β-ocimene，α-curcumene，which shows certain inhibitory effect on in vitro proliferation of A 549 cells.