Abstract: Chlorate resistance is one of the reliable characters in Indica/Japonica classification. To understand the mechanism of chlorate resistance is very important for revealing the evolutionary mechanism of Indica/ Japonica differentiation. In this study, 4 Indica materials and 4 Japonica materials were used to detect the correlation between nitrate reductase (NR), nitrite reductase (NIR) and chlorate resistance. NR activity showed a significantly positive correlation with chlorate response index (r=0.716*, 0.046 significance level), and the correlation of NIR activity and chlorate response index was not significant (r=- 0.628, 0.096 significance level), whereas the ratio of NIR/NR showed a very significantly negative correlation with chlorate response index (r=- 0.879**, 0.004 significance level). When NR activity was inhibited partially, NIR activity was changed with a significantly negative correlation with chlorate response index (r=-0.709*, 0.049 significance level). Therefore, we deduce that the ratio of NIR/NR may be a strong influence on chlorate resistance, and a great value for revealing the mechanism of chlorate resistance of Oryza sativa.