Abstract: Objective：To investigate the effects of traditional Chinese medicine monomer Toonaciliatin A on proliferation and apoptosis of human gastric cancer cell line MKN45 in vitro and in vivo. Methods：MKN45 cells were cultured in vitro and treated with Toonaciliatin A (15, 30, and 60 μg/mL) for 48 h. Then the cell viability was measured by MTT assay. The apoptosis were determined by flow cytometry using Annexin VFITC/PI assay. The BALB/c nude mouse xenograft model of MKN45 cells was established and administrated with Toonaciliatin A (10, 20, and 40 mg/kg) for 21 days, then the weights of xenograft tumors and mouse were calculated, and the expression of caspase3 protein in tumor tissues was detected by immunohistochemical method. Results：As compared with the control group, Toonaciliatin A inhibited effectively the proliferation of MKN45 cells, and induced apoptosis of MKN45 cells in vitro (P<0.05). In the nude mouse xenograft model of MKN45 cells, the tumor weight of Toonaciliatin A treatment group was significantly decreased as compared with that of the control group (P<0.05). Toonaciliatin A induced the expression of caspase-3 protein in vivo (P<0.05). Conclusions：Toonaciliatin A is an effective antitumor Chinese medicine monomer.