The present study suggests that nimbolide inhibited the proliferation of bladder cancer cells through G2/M phase cell cycle arrest mediated by both the Chk2-Cdc25C-Cdc2/cyclin B1-Wee1 pathway and Chk2-p21WAF1-Cdc2/cyclin B1-Wee1 pathway. The gene discussed is CCNB1; the disease is urinary bladder cancer.