Objective: To investigate the role of p21 in the regulation of bufalin on the mesangial cell (MC) proliferation. Methods: The effects of different concentrations of bufalin (0.01,0.04,0.08,0.16 μmol/L) on MC proliferation induced by PDGF-BB(20 ng/ml) was evaluated by MTT test; Flow cytometry and RT-PCR were utilized for cell cycle analysis and p21 DNA expression,,respectively. Western blot was used to analyze protein expression of p21. Results: After twelve hours of PDGF-BB stimulation, MC was cultured with different concentrations of bufalin from 0.04 μmol/L to 0.16 μmol/L for 24 hours, respectively. Proliferation of GMC induced by 20 ng/ml PDGF-BB was inhibited significantly by bufalin in a dose-dependent manner(P < 0.05). Similar results were observed in 0.08 μmol/L bufalin, which blocked the PDGF-BB-induced progression from G0/G1 to S phase of the cell cycle in synchronized cells(P < 0.05). In addition, we found that bufalin reversed PDGF-BB-induced down-regulation of cyclin-dependent kinase inhibitor p21 on both gene and protein level(P < 0.05). However, 0.08μmol/L bufalin had no effects on normal mesangial cell proliferation (P > 0.05). Conclusion: Bufalin may inhibit the PDGF-BB-induced MC proliferation by increase of p21 expression.