Abstract: Kernel number was the main factor affecting maize yield.Nitrogen supply affected carbon-nitrogen(CN) metabolism in maize,then affected kernel number and yield.The objectives of this work were: 1)to evaluate the effect of nitrogen on yield and its components,and to determine the relationship between yield and kernel number;2)to analyze C-N metabolism in maize,and explore the possible reasons affecting kernel set.A field experiment with summer maize(Zea mays L.) hybrid Zhengdan 958 was carried out at Wuqiao experiment station of China Agricultural University in 2004.The plant density was 67500 plant/ha.Four basic nitrogen rates(0,120,180,240 kg/ha) and two top dressing rates(240,300 kg/ha) in silk stage were set up.Grain yield and its components,the content of soluble sugar and nitrogen,the main enzymes that affected C-N metabolism such as nitrate reductase(NR),grutamine synthetase(GS),and sucrose phosphate synthetase(SPS) in maize were studied.The results showed that nitrogen affected maize yield mainly through affecting kernel number.Nitrogen supply could evidently improve the activities of enzymes NR,GS,and SPS in ear leaves,accelerate CN metabolism during kernel set stage,promote assimilate accumulation and distribution,and increase kernel number and yield.The results suggested that the nitrogen supply rate 180 kg/ha showed the best result on kernel number and yield in maize,while the nitrogen supply rate 240 kg/ha had negative effect.Under the prerequisite of nitrogen adequate supplied before silk stage,top dressing nitrogen in silk stage had negligible effects on yield.