This study was carried out in order to investigate the germination inhibiting substance extracted from the leaves of 165 species belonging to 49 families, in Korea. Several grasses important for erosion control are used for germination testing. The results are summarized as follows; 1. Lettuce, radish and italianraygrass were used for germination testing. The extracts tested were obtained from 165 species. The extracts from Pinus densiflora, Pinus thunbergii, Abies holophylla, Chamaecyparis picifera, Salix koreansis, Prunus sargentii and Styrax japonica were more effective in inhibiting germination than others. 2. Several variation in the germination rate was observed. The extracts from leaves, which were harvested in the fall, inhibited the germination stronger than the ones of the spring. 3. Among the seven active species, Styrax japonicc showed the strongest inhibiting effect, followed by Salix koreansis, Prunus sargentii, Abies holophylla, Chamaecyparis picifera, Pinus thunbergii and Pinus densiflora in ascending order. 4. In order to investigate the germination inhibitor of Styrax japonica leaves, solvent fractionation from these was undertaken, giving petroleum ether fraction and water fraction. The petroleum either fraction was not effective, The effective water layer was column-chromatographed, combined with thin layer-chromatographying. The most effective germination inhibitor has Rf 0.72 in fraction III of the column-chromatography with solvent of chloroform: MeOH (97:3, v/v). 5. In the germination testing with various concentrations of the effective substance, the germination of lettuce and Themeda japonica was fully inhibited at the level of 100ppm and 250ppm, respectively. 6. According to results of the physical and chemical analysis, the effective substance was proven belong to Aryl-aliphatic alcohol. 7. The germination of several soil conservation grass seeds are inhibited by water, ether and methanol extracts from Styrax japonica leaves, therefore it is supposed that the forest ecosystem and soil conditions, at treated or designated-land for erosion control, are disadvantageously effected by the formation of Styrax japonica community.