Abstract: In order to determine the effect of exogenous H₂O₂ on oat (Avena sativa) seed vigor during the germination process, oat seeds were pre-treated with 5 different concentrations of H₂O₂ (0, 0.24, 0.48, 0.96, 1.92, 3.84 mol·L^-1) for 0 (CK), 3, 6, 9 and 12 h. The performance indices measured included germination percentage (G_p), germination index (G_i), mean germination time (MGT) and seedling vigor index (SVI). The results showed that with high H₂O₂ concentrations the G_p, G_i and SVI of oat seeds decreased but their MGT increased. The effects of H₂O₂ on changes in G_p, G_i, MGT and SVI were significantly related to treatment concentrations, times and their interactions. The G_p, G_i and SVI of oat seeds reached their minimum level and MGT its maximum level after treatment with H₂O₂ concentration of 3.84 mol·L^-1 for 12 h. These results indicate that the higher the H₂O₂ concentration and the longer of treatment time, the more serious the loss of oat seed vigor. The lipid peroxidation damage caused by high concentration H₂O₂ treatments might be a major reason for the loss of seed vigor. This study suggests that the application of H₂O₂ in seed pre-treatment should be prudently undertaken.