Abstract:To determine the effects of nitrogen rates on activities of nitrogen metabolism related enzymes and grain protein quality of wheat, the field experiment was conducted with a strong gluten wheat cultivar Jimai 20 in 2003–2004 and 2004–2005 growing seasons. Five nitrogen rates with N0, 96, 168, 240, 276 kg/ha and four nitrogen rates with N 0, 96, 168, 240 kg/ha were designed respectively. The results of two years show that: In proper range of nitrogen rate, the total protein content in grain, No. of spikes and grains per hectare, grain yield, protein yield increased significantly with the increase of nitrogen rate; Continue to increase nitrogen rate , grain yield decreased significantly. No. of spikes , grains per hectare and protein yield decreased or had no significant differences. In the 2004–2005 growing season, with the increase nitrogen rates from 0 to 168 kg/ha, glutamine synthetase (GS) activities in flag leaves after anthesis, endopeptidase (EP) activities in flag leaves at 21 days after the anthesis, free amino acid contents in flag leaves, gliadin content in grain, high molecular weight glutenin subunits (HMW-GS) and low molecular weight glutenin subunits (LMW-GS) contents, HMW-GS / LMW-GS ratios, total protein contents in grains, No. of spikes and grains per hectare, grain yields are all increased significantly, while the dough development time and dough stability time are not extended. However, with 240 kg/ha nitrogen applied, GS activity is not significantly changed, EP activity in flag leaves at 21 days after the anthesis, ω-gliadin, α-gliadin, HMW -GS, LMW-GS and total protein contents in grains are still increased significantly, the dough stability time is extended, and the grain yield is decreased significantly. It is indicated that the excessive nitrogen application is useless for nitrogen assimilation and grain yield. There are indirect regulations of GS and EP on synthesis of protein components in grain. Improving GS activity in flag leaves after the anthesis and EP activity in flag leaves at the late filling stage are conducive to the accumulations of HMW-GS and LMW-GS, and the increase of HMW-GS / LMW-GS ratio. The synchronization increases of grain quality and yield are achieved by increasing the number of spikes and grains per hectare and the storage of the nitrogen and photosynthetic products in grain per unit area when appropriate amount of nitrogen applied. Under this experimental conditions, the optimum nitrogen rate for high quality, high yielding and high efficient production is 168–240 kg/ha.