The understory vegetation of a Chamaecyparis obtusa plantation is relatively sparse at a valley in Sangkwan-my6n, Wanju-gun, Chiinbuk, Korea, and that of the pure C. obtusa plantation is more sparse than in the C, obtusa-Larix leptolepis plantation. In order to investigate the causes of this difference, this study was carried out both in the field and in the laboratory. Total of 109 taxa, comprising 90 genera and 53 families, were identified in the plantation. But the average number of species above 10% frequency was only 27, and both the number of species and plants per quadrat in the pure C. obtusa plantation were lower than those in the C, obtusa-L. leptolepis plantation. Light intensity, soil pH, and the difference of soil minerals were supposed to be parts of the causes. Soils, litter extracts of C. obtusa and L. leptolepis, and leachates collected by vermiculite beneath C. obtusa canopy for 1 year were tested for the effect on germination and seedling growth of both inside species and outside species in the laboratory. Germination and seedling growth of outside species were more suppressed than those of inside species. Therefore it was found that allelopathic effect of C. obtusa would be responsible for the sparse understory vegetation in the C. oblusa plantation.