In order to clarify the mechanism of high dry matter production in napier grass (Pennisetum purpureum Schmach), a tall-grass-type plant with perennial growth, the dry matter weight of plant, growth parameters, light extinction coefficient (K) of canopy and photosynthetic characteristics of single leaf were investigated and compared with those of maize plant (Zea mays L.). Both species were grown under the high planting density and sufficiently fertilized field conditions. Their dry matter production was very high, and the highest CGR (Crop Growth Rate) reached 71.1gm^<-2> day^<-1> in napier grass. The causes that napier grass had a high productivity are presented as follows ; 1) Under the climatic condition of Fukuoka (33.36 NL, 130.23 EL), LAI of this species during the growth period was 6.4 on the average and the optimum LAI reached 11.77 at the late growth stage 70 to 80 days after transplanting. The light extinction coefficient (K) of napier grass was lower than that of maize, also having a decreasing trend with growth. A joint effect of long period growth, high LAI and the good penetration of light into the canopy is regarded as the main cause for the high productivity of napier grass. 2) In addition to this, photosynthetic rate of leaves positioned in upper layers in the canopy was higher in napier grass than maize; this becomes a cause of high production of napier grass. The leaf of napier grass was more adaptable to shadings, so that the light utilization efficiency of photosynthesis increased in leaves grown in lower layers of the canopy. This is also a contributing factor for increment of canopy photosynthesis in napier grass.