In young sugi (Cryptomeria japonica) plantations in mountainous regions of Kyushu, broadleaved trees, including useful species such as mizunara (Quercus mongolica var. grosseserrata) and buna (Fagus crenata), have intensively regenerated after final weeding, and thereby mixed stands with the natural broadleaved trees have been distributed. The numbers of the surviving sugi have decreased and its tree height growth has been poorer with higher altitude in the same watershed, whereas the numbers of mixed broadleaved trees have increased with higher one and their tree height growth has hardly differed by altitude. Consequently, the competition between the planted sugi and mixed broadleaved trees tends to intensify with higher altitude, and the unsuccessful sugi plantations are hence distributed in the upper parts of the mountain slopes and show the various stand composition caused by the mixed numbers of the broadleaved trees, the stratification of the whole trees and the intensity of the competition between the both trees. These young sugi plantations should be induced to form mixed forests with natural broadleaved trees including some useful species, because it is impossible to develop the young plantations into uniform sugi ones.