AIM: To investigate the influence of tumor necrosis factor-α (TNF-α) induced apoptosis through knocking down Bax gene by RNA interference (RNAi) in human alveolar type II epithelial cells (A549). METHODS: A549 cells were cultured in vitro and divided into 4 groups according to RNAi treatment: control group, TNF-α treated group, Bax siRNA group and control siRNA group. Chemically synthesized small interfering RNA (siRNA) directed against human Bax gene was transfected into A549 cells by cationic liposome. The effect of RNAi was investigated by reverse transcription PCR, Western blotting and immunohistochemisty, and the rate of apoptosis was investigated by flow cytometry. RESULTS: Bax gene was knocked down effectively in Bax siRNA group (P<0.05). Apoptosis was induced by TNF-α in TNF-α treated group and control siRNA group. However, it was abolished in Bax siRNA group by the downregulation of Bax gene compared to TNF-α treated group and control siRNA group (P<0.05). CONCLUSION: This result suggests that Bax gene has a significant role of pro-apoptosis in A549 cells and knocking down Bax gene by RNAi can effectively inhibit TNF-α-induced apoptosis in A549 cells.