Objective To evaluate the effects of small interfering RNA (siRNA) against peptidylarginine deiminase 4 (PADI4) gene on apoptosis of fibroblast-like synoviocytes (FLS) from synovium of rheumatoid arthritis (RA).Methods The siRNA targeting PADI4 was constructed and transfected into FLS cells in RA via LipofectamineTM 2000.The expression level of PDAI4 mRNA was detected by using real-time quantitative polymerase chain reaction (real-time PCR).The protein expression of PADI4,CyclinB1 and P21 was detected by Western blotting.The apoptosis of FLS cells in RA was examined by flow cytometry.The levels of IL-1β were detected by ELISA.T-test was used for statistical analysis.Results siRNA-PADI4 efficiently down-regulated the PADI4 expression compared with control group,1.00±0.20 vs 0.38±0.20 (t=9.607,P＜0.01),0.39±0.23(t=8.394,P＜0.01).FCM analysis showed that the percentage of apoptosis cells in PADI4 siRNA group in FLS was (5.4±0.6)％ (t=-19.223,P＜0.01) and (6.1±0.6)％ respectively (t=-24.229,P＜0.01),which was significantly higher than that in the control group in FLS (1.6±0.3)％.The expression of CyclinB1 protein was decreased,and P21 increased.The concentrations of IL-1β in culture medium of the transfected group were (26.8±0.7) ng/ml (t=-10.747,P＜0.01) and (27.7±0.7) ng/ml (t=-10.967,P＜0.01),higher than the control group [(23.9±0.7) ng/ml].Conclusion After being transfected with PADI4 siRNA,the apoptosis of FLS cells in RA is increased.Our results have demonstrated the potential role of CyclinB1 and P21 in PADI4 signaling.