AIM: To investigate the effects of nicotinamide (NAM) on bevacizumab (BEV)-induced epithelial-mesenchymal transition (EMT) of human retinal pigment epithelial cells (ARPE-19) and the underling mechanisms. METHODS: ARPE-19 cells were treated with BEV for 24, 48, and 72h, and the variation degrees of EMT-related markers (fibronectin, α-SMA, vimentin, and ZO-1) were assessed by Western blotting to select the optimal treatment time point which exhibited the most obvious changes of EMT-related markers for the subsequent experiments. Furthermore, NAM was added to the medium, the mRNA and protein levels of the EMT-related markers were then measured. The accumulation of reactive oxygen species (ROS) and H2O2 and the total antioxidant capacity (TAC) of the cells were also measured to evaluate the level of oxidative stress. RESULTS: After being treated with BEV for 72h, the protein expression levels of EMT-related markers in ARPE-19 cells showed significant changes. Meanwhile the levels of ROS and H2O2 were obviously increased, and the TAC of ARPE-19 cells was decreased. Totally 72h was chosen to be the optimal treatment time point in subsequent experiments. Furthermore, NAM inhibited BEV-induced EMT by downregulating fibronectin, α-SMA, and vimentin and upregulating ZO-1, decreased the accumulation of ROS and H2O2, and enhanced TAC in BEV-treated ARPE-19 cells. CONCLUSION: This study demonstrates that NAM suppressed BEV-induced EMT in ARPE-19 cells by attenuating oxidative stress. Hence, NAM may be a potential therapeutic agent for alleviating neovascular fibrosis of the ocular fundus after anti-vascular endothelial growth factor therapy.