A maize strain of yellow sweet was intercropped between the four species of fast-growing trees each at Bayog in Los Ba?os, the Philippines, to examine the effects of intercropping system on maize growth and yield. The tree species planted were: Gmelina arborea, Paraserianthes falcataria, Acacia mangium and Eucalyptus deglupta. Two spacings, or 2 × 2 m and 2 × 4 m, were employed in planting trees, and two spacings, or 0.25 × 0.75 m and 0.25 × 2.0 m, were given for a yield trial of maize. In relation to the environmental conditions for growing maize, relative light intensity on the forest floor and basal area were greatly affected by trees' crown, which was closely associated with their branch spread. The largest crown radius took place in G. arborea up to the level of 6 m in plant height. However, it was replaced by P. falcataria beyond that level. Under the unclosed intercropping condition, soil temperature of the interrows was lower by 5℃ than that of the bared ground in open fields. Plant height and above-the-ground biomass of maize were lowered in accordance with decreasing relative light intensity. Grain yield of maize in the intercropping system with a basal area of less than 0.5 m²/ha was of the same level as that in the monoculture with a thin density, i.e. 0.25 × 2.0 m. In order to increase crop yields in agroforestry, solar energy has to reach the lower layer of the system as much as possible. Towards this end, it is necessary to remove and/or reduce part of canopy of the upper layer by thinning, pruning and trimming.