Abstract: The effects of N fertilizer rates on some physiological characteristics of ratoon rice were studied.The results showed that the activity of nitrate reductase (NR), chlorophyll contents and net photosynthetic rate in the leaves of ratoon rice increased as nitrogen application rate (57.5-287.5kg·hm-2) was enhanced.The biochemical analysis indicated that the optimum nitrogen application rate also increased the activities of cell defense enzymes, thereby reduced menbrane detriment by active oxygen species produced in the process of senescence, consequently, protected photosynthetic function of green leaves in rice and enhanced root activity, in turn delayed senescence and increased yield.