Objective： To investigate the chemopreventive effect of tamoxifen combined with a COX-2 selective inhibitor， celecoxib， on breast cancer in rats chemically induced by 7，12-dimethylben（a）anthracene （DMBA）. Methods：DMBA was irrigated into the stomaches of SD female rats to build breast cancer model. A total of 120 rats were divided into four groups： control group， tamoxifen group， celecoxib group and combined group. The incidence rate， latent period， number and volume of breast cancer were detected and analyzed. Results：The tumor incidence rate of tamoxifen group （48.15%， 13/27） and celecoxib group （50.00%， 14/28） were lower than that of control group （85.71%， 24/28）， but higher than that of combined group （21.43%， 6/28）. The tumor’s latent period of tamoxifen group （97.54±1.85 d） and celecoxib group （96.79±2.89 d） were longer than that of control group （89.50±5.99 d）， but shorter than that of combined group （103.67±3.39 d）. The average tumor number of tamoxifen group （1.77±0.73） and celecoxib group （1.71±0.61） were less than that of control group （3.50±1.62）， but more than that of combined group （1.17±0.42）. The average tumor volume of tamoxifen group （1.78±0.71 cm3） and celecoxib group （2.05±1.04 cm3） were smaller than that of control group （6.42±3.96 cm3）， but bigger than that of combined group （0.71±0.96 cm3） （P < 0.05 respectively）. Conclusion：Celecoxib and tamoxifen are effective drugs in preventing the occurrence of rat breast cancer chemically induced by DMBA. Furthermore， combination of them has better chemopreventive effect.