Abstract: The amount of N in soil had a direct influence on the protein content in the maize. Application of N coordinated with other nutrients (such as P, K, Mg) could not only increase the grain yield, but also the protein and lysine content. The activities of nitrate reductase (NRase), glutamine synthetase(GS), and glutamate dehydrogenase(GDH) in the leaves and grain of maize were close related to protein synthesis in kernel. However, few studies were carried out on the effect of N application on the quality constituents (protein and its constituents, amino acid, fat and its constituents) of maize. In order to learn the dynamic change of protein, its constituents and activities of relevant enzymes in the grain formation process, a pot culture experiment was carried out. Two types of maize (Zea mays) cultivars with different oil content (Yedan 22, a common cultivars and Gaoyou 115 with high oil content) were selected as experimental materials. The results showed that the content of grain protein and its constituent of Gaoyou115 were significantly higher than that of Yedan22. Among which, albumin and gliadin content, which was significant higher than those of Yedan22, increased with the increment of N supply . Under different conditions of nitrogen supply,the activities of NRase,GS in leaves and GS,GDH in grain of both cultivars took on a same tendency. The activity of NRase decreased all the time from prophase of grain filling stage to mature stage, while the activities of GS and GDH took on a singlet curves,with the peak appearing between 20 days and 30 days after pollination. Increasing the supply of N was favorable to maintaining the activities of these enzymes at high level. Comparing with Gaoyou115, The activity of NRase in leaves of Yedan22 was higher,while the activities of GS in leaves and GDH in grain was significant lower. This could results in a lower protein content in grain of Yedan22 comparing with Gaoyou115.