The shortage of leaf area restrains improvement of corn yield under the routine method. In the present paper, as a basic research for the study of increment of dry matter production, the growth of corn seedlings under extremely dense population (plots consist of 5cm and 7.5 cm of planting interval) was investigated on experimental scale. Large seed variety of Koh No. 7 was planted five times throughout growing season, and each experiment was started from five-leaf stage until lodging. Results are as follows. 1. The total amount of five final standing crops attained 3349.2 g/m2 and 2651.4 g/m2 at 5 cm plot and 7.5 cm plot, respectively. 2. Mean CGR throughout the growing season were 20.2 g/m2·day and 17.3 g/m2·day and which efficiency for sunlight utilization were 2.11% and 1.80% at 5 cm plot and 7.5 cm plot, respectively. 3. Maximum LAI 18.7 and maximum CGR 54.7g/m2·day for one-week period were obtained at higher population density. The outline of the discussion about the mechanism of ascent of CGR under extremely high LAI can be expressed as follows. With the increment of LAI, adaptation to low light intensity such as decline in light extinction coefficient K and ascent of specific leaf area was occurred. As the effect of these two factors, efficiency for sunlight utilization ascended and respiration rate descended under low light intensity. Therefore, high CGR was attained.