Abstract: Suitable nitrogen application rate can significantly increase grain filling rate and yield and improve nutritional quality. Denghai 518 (DH518) and Zhengdan 958 (ZD958) were used as experimental materials in this study. A field experiment with four treatments, no nitrogen treatment (N₀), decrement nitrogen application rate (N₁, 129 kg N·hm^-2), suitable nitrogen rate (N₂, 184.5 kg N·hm^-2) and excessive nitrogen rate (N₃, 300 kg N·hm^-2), was conducted to explore the effects of nitrogen fertilization on grain filling parameters and nutritional qualities of summer maize. Results showed that grain filling characteristics, grain dry weight, and yield in N₀ treatment was decreased. With increasing nitrogen application rate in the suitable range, average filling rate, grain dry weight, and yield increased. Grain yield of two hybrids in N₁ and N₂ treatments was higher than that of N₀ by 16.4%-57.2% and 35.8%-65.1%, respectively. Grain protein, soluble sugar, starch contents and the ratio of amylopectin and amylase contents were lower and crude fat content was higher in N₀ treatment. Grain protein content, soluble sugar content and starch content in N₂ of DH518 were higher than that of N₀ and N₁ by 32.5% and 6.5%, 19.9% and 9.5%, 8.9% and 5.2%, and the ratio of amylopectin and amylose contents was increased. Grain protein, soluble sugar and starch contents in N₂ of ZD958 were higher than that of N₀ and N₁ by 16.9% and 7.8%, 30.5% and 14.8%, 11.5% and 5.7%, and the ratio of amylopectin and amylase contents was increased. Crude fat content in N₂ of both hybrids decreased significantly by 4.8%-12.3% than that of N₀ and N₁. However, yield and nutritional quality was increased in N₃ treatment than that of N₂. Our results suggested that suitable nitrogen rate could enhance grain filling, grain weight, and grain nutritional quality.