Abstract: The present study was carried out during 1989—1990 in high yield summer corn field. The results were as follows:Diural course of canopy apparent photosynthesis (CAP) rate was a unimedal curve(11:00—12:00). The relationship between light intensity and CAP rate showed rectangular hyperbola. The light compensition point of CAP was about 4000 Lx. No light saturation point observed under field condition. CAP increased with CO2 concentration from 50 to 900ppm. The ratio of CO2 assimilated to that released from the soil was 11.68% througout growth period. Photosynthetic rate of ear leaf was higher than the sixth leaf and the leaf next to the top leaf when leaf was expose fully to light, but there was no difference among cultivares and plant densities. Leaf photosynthetic rate was in the order of top leaves>middle leaves>base leaves under natural state and vertical leaf type cultivar>horizonal under high density condition.