Objective: To study the killing effects of bak gene on multi-drug resistant (MDR) bladder cancer cells and the mechanisms. Methods: Bak gene was transfected into MDR bladder cancer cells by liposome.The expression of bak and Bcl-2 mRNA was detected by in situ hybridization. The expression of bak and Bcl-2 proteins was detected by SABC immunohistochemistry. The growth rate of human bladder cancer cells was studied by constructing the growth curve, cell apoptosis was measured by flow cytometry, and the morphology of cells was observed by fluorescence stain. Results: The expression of bak mRNA was positive in EJ/bak cells (P＜0.05). Bak protein expression of EJ/bak cells was positive and Bcl-2 protein expression was decreased (P＜0.05). The growth of MDR bladder cancer cells was significantly inhibited after bak gene was transfected (P＜0.05). Apoptosis cells were increased significantly. The apoptosis rate was 35%. Apoptotic bodies can be found in these cells by fluorescence stain. Conclusion: Bak gene could inhibit the growth of MDR bladder cancer cells effectively. Inducing cell apoptosis by down-regulating the expression of Bcl-2 gene might be one of its mechanisms.