Abstract: Objective：To study the effect of Periplocin from Cortex Periplocae (CPP) on cell cycle of MCF-7 cells and expression of p21^WAF1/CIP1, and demonstrate the possible mechanism of anti-tumor. Methods：Inhibitory effects of CPP in different concentrations (1.25, 2.50, 5.00, 10.00, 20.00 ng/ml) and different time (24, 48, 72 h) on proliferation of MCF-7 were detected by MTT. The changes of cell cycle of tumor cells treated with CPP under different concentrations (2.50, 5.00, 10.00ng/ml) and various time (6, 12, 24, 48, 72 h) were detected using flow cytometry,respectively. Expression of cell cycle associated gene p21^WAF1/CIP1 was assessed by semi-quantitative RT-PCR and SP immunocytochemistry method. Results：A dose and time-dependent proliferation inhibition of CPP was demonstrated in MCF -7 with IC₅₀ value (for 48h) of (4.88±0.16) ng/ml. Compared to control group, the number of G₀/G₁ phase cells increased markedly, but which of S and G₂/M phase cells decreased, after treatment with CPP for 24 hours, the difference was significant (P<0.05); in 5.00 ng/ml group, the proportion of tumor cells in the G₀/G₁ phase was increased from (49.33±3.25)% to (79.47±2.40)%, the proportion of S and G₂/M phase cells was decreased from (28.47±1.59)% and (22.20±2.09)% to (10.13±3.26)% and (10.40±1.41)%,respectively. p21^WAF1/CIP1 mRNA level increased obviously in MCF-7 cells exposed to CPP, the ratio of p21^WAF1/CIP1 to β-actin was much higher, compared with that in control group (P<0.05). The result of immunocytochemistry indicated that p21^WAF1/CIP1 protein expression increased obviously in MCF-7 cells exposed to CPP in concentration-dependent manner. p21^WAF1/CIP1 protein showed stronger positive staining in cells with 10.00 ng/ml CPP treatment. Conclusion：CPP has marked anti-tumor effect in vitro, its curative dose is small, the IC₅₀ was (4.88±0.16)ng/ml. CPP could hinder the cell cycle of MCF-7 cells at G₀/G₁ phase. CPP elevated mRNA and protein expression of cell cycle associated gene p21^WAF1/CIP1. The result suggested that blocking the cell cycle may be one of its anti-tumor mechanisms in vitro.