Abstract: Objective To investigate the chemopreventive effects of mustard seed（MS） on azoxymethane （AOM） induced colorectal tumor in mice and to explore its anticancerrelated mechanisms.Methods A total of 60 mice of Kunming species were randomly divided into 4 groups of 15 each： AOM alone， AOM+5％MS， AOM+10％MS and the control group （normal saline）. Colorectal tumorigenesis was induced by injecting AOM 10 mg／kg， subcutaneously， once a week for three weeks. Different doses of MS were given in diet during the study. Thirty-six weeks later， mice were sacrificed under cervical dislocation. The large intestine was isolated and flushed with icecold normal saline. The site， size and number of tumors were recorded. The incidence， the average number and the inhibitory rate of colorectal tumor in each group was determined. Tumor tissues were fixed, dehydrated and paraffinembedded. The histological type was determined by HE staining. Expression of PCNA protein in tumor tissue was detected by immunohistochemical staining and tumor cell proliferation index （PI） was calculated. Apoptosis of tumor tissue was detected by TUNEL staining and tumor cell apoptosis index （AI） was calculated too. Results No tumor was found in control group. The colorectal tumor incidence， as compared to AOM model group （86.7％）， was reduced to 60.0％ by the 5％ diet of mustard seeds and to 41.7％ by the 10％ diet of mustard seeds. The tumor incidence rates of the three groups were significantly different（P=0.048）， with the highest incidence rate in AOM model group， followed by 5％MS intervention group， 10％MS intervention group minimum. Tumor burden （tumors／mouse） was 2.20±1.21 in the AOM model group. Compared with model group， tumor burden was 1.07±1.10 in 5％MS intervention group， and 0.67± 0.89 in 10％MS intervention group（P＜0.05）. PI in 10％MS intervention group was 32.0±3.9， which was lower than that in the AOM model group（59.9±4.4） and 5％MS intervention group（41.7±4.9）， with significant differences（P＜0.05）. AI in 10％MS intervention group was 15.0±2.4，which was higher than that in the AOM model group （6.9±1.4） and 5％MS intervention group（9.3±1.5）， with significant differences （P＜0.05）. Conclusion The mustard seeds possess chemopreventive activity against colorectal cancer induced by AOM in mice， and can reduce the incidence of tumor and average number of tumor. The mustard seeds can reduce the expression of PCNA in colorectal tumor tissue, inhibit tumor cell proliferation and induce apoptosis of tumor cells.