Abstract: Increased plant density decreased both shoot and root growth of maize plant. The causal relationship of the decreased shoot and root growth was studied through a field experiment with 2 factors (i.e. plant density and nitrogen application rate) and 4 replications using DH3719, high-yield maize（Zea mays L.）genotype. The results showed that the plants with higher plant density had higher LAI. On the other hand, the total leaf area, shoot and root dry weight, stem diameter, total root length and nitrogen content of the individual plant with higher density were lower than those of the plant with low plant density, especially after six-leaf stage, when plants began to show competition on above growth space. The changes in total leaf area and root length of single plant during the growth period showed the similar pattern. Also, shoot/root dry weight ratios of the different treated plants at each growth stage were not different. Total N accumulation in single plant was decreased when two leaves above ear leaf were cut off or the up whorl of the shoot-borne roots was removed at the tasselling stage. In comparison, less N was taken up in the former treated plants than that in the later treated ones, in spite of the less influence of cutting off leaves than removing the up whorl of the shoot-borne roots on root dry weight. In conclusion, higher plant density caused increased competition on above ground space between plants, resulted in decreased shoot growth and therefore decreased root growth.