Abstract: In order to clarify the effects of different levels of nitrogen on dry matter translocation,grain filling and activities of enzymes involved in starch synthesis of wheat, to improve nitrogen use efficiency, to reduce the effect of excessive application of fertilizer on environment pollution and to increase crop yield, a field experiment was carried out in the experimental farm of Henan University of Science and Technology under different nitrogen treatments,120 kg/ha, 180 kg/ha, 240 kg/ha and 300 kg/ha. The wheat cultivar, Yumai 49-198 was selected for the study. The results indicate that the increase of nitrogen input within a certain extent could increase dry matter weights of stem and sheath at the heading and harvest stages, exporting amount of stems and sheath and export and transform percentages of the matter after the heading. These indexes are highest under the N3 treatment and are lowest under the N1 treatment. Increasing proper nitrogen could improve grain growth potential, the maximum rate of grain filling and mean rate of grain filling, and shorten the active grain-filling period and the time of reaching the maximum grain-filling rate. The N3 treatment is the best, its maximum grain filling rate and the average grain filling rate are 0.55710-2 and 0.373 10-2 g/(grainsd), respectively. The N4 treatment is the worst, its maximum grain filling rate and the average grain filling rate are 0.40610-2 and 0.272 10-2 g/(grainsd), respectively.Increasing proper nitrogen could increase activities of key enzymes of wheat grain including soluble starch synthase (SSS), ADP glucose pyrophosphorylase (ADPGPPase) and soluble starch branch enzymes during the grain filling. Compared to the N4 treatment, the enzyme activities are the highest under the N3 treatment and are the lowest under the N4 treatment. Maximum enzyme activities of SSS, Q and ADPGPPase under the N3 treatment are increased by 22%, 23.2% and 9.5%. Dry matter weights of stem and sheath at the heading,exporting amounts of stems and sheath, export and transform percentages of the matter after the heading are positively significant or very significant correlation with the grain yield. This experiment shows that higher export and transform percentages of stem and sheath, rate of grain filling and activities of key enzymes of wheat grain are the physiological basis for higher yield under a appropriate nitrogen level.