Abstract: Effects of nitrogen application rates(N 0, 75, 150, 225, 300 and 375 kg/ha)on chlorophyll content, net photosynthetic rate, nitrate reductase activity, contents of soluble sugar and soluble protein in main stem leaves, yield, fiber quality of hybrid cotton were studied under high yield conditions. The results show that above photosynthetic and physiological indices are increased at the earlier stages of cotton and decreased at the later stages. The increasing trends of these indices are under the N rates in range of 0-300 kg/ha, while compared with N 300 kg/ha application these indices are decreased obviously at the later stages under N 375 kg/ha application. The results of the comparisons among different N rates indicate that photosynthetic and physiological characteristics have no significant differences on July 5, but at the middle and late growth stages, the N rates have strong effects on them. The photosynthetic and physiological characteristics are increased with the increase of N rates in the range of 0-225 kg/ha, but when N 300 or 375 kg/ha is applied, the photosynthetic and physiological indices are no longer increased significantly. On the contrary, the photosynthetic and physiological indices are decreased when N 375 kg/ha is applied. The total boll number, rotten boll rate and biomass are increased when more N is applied, while the harvest index is decreased. The boll weight is increased with the increase of N rates in the range of 0-300 kg/ha, while it is decreased when the N rate is up to 375 kg/ha. The N rates have little effects on the cotton lint percentage. Seed cotton yield is increased with the increase of N rates in the range of 0-300 kg/ha, but it is decreased by 392％when N rate is 375 kg/ha. As to the fiber quality, applying nitrogen could improve the fiber length and fiber strength significantly; however there are no significant differences among different N rates. It is concluded that the leaves of hybrid cotton have higher photosynthetic capability and longer photosynthetic duration when N 225-300 kg/ha are applied which is benefit to increase yield under this experiment conditions.