Abstract: To determine the effects of nitrogen fertilizer on fresh ear yield, quality and enzyme activity associated metabolism of sweet corn, a field experiment was conducted with two corn cultivars, Yangtian 2 and Chaotian 135 There were four nitrogen rates, N 0, 75, 225 and 375 kg/ha. The results show that the fresh ear yield of Yangtian 2 and Chaotian 135 are significantly increased under the nitrogen fertilization, and the yield is highest under N 225 kg/ha application. Compared with nonitrogen treatment, the yields of the treatment with N 225 kg/ha of the two cultivars are increased by 65.15% and 99.61% respectively. The contents of soluble sugar and sucrose at the grain filling stage are single peaked curves. In a proper range of nitrogen level, the contents of soluble sugar and sucrose are increased significantly with the increase of nitrogen level, and the contents are highest under N 225 kg/ha application. The rates of nitrogen fertilization have great effects on texture properties of sweet corn, the hardness of grains is significantly increased with the amount of nitrogen fertilizer increased, and the fracturability is significantly increased at first and then decreased with the increase of N fertilizer application. The value of fracturability reaches the highest level under N 225 kg/ha application. The dynamic changes of activities of sucrose phosphate synthase (SPS) and sucrose synthase (SS, synthesis) are similar to the sucrose accumulation. With the proceeding of grain filling, the activity of SS (cleavage) is gradually increased. Appropriate N fertilizer could increase the activities of SPS and SS (synthesis) and improve the content of sugar and nutrition quality of sweet corn.