Structure, composition and their dynamics of plantations of Cryptomeria japonica (ca. 20 years) with colonizing native trees were investigated along Okuakaishi forest road in Shirakami mountainous district from 1998 to 2002. Stem density of Cryptomeria varied greatly among study plots ranging from 350 to 2300 no. ha^-1, and their basal area were also variable from 3.6 to 41.9 m²ha^-1. The plots with lower stem density and basal area did not show any tendency of improvement of stand conditions. Because the most important factor of mortality was damages by heavy snow, above spatial variation of stand growth was considered to result from spatial heterogeneity of snow depth during winter. Native broad-leaved trees colonized the plantations with basal area ranging from 0.0 to 15.4 m²ha^-1; their basal area was negatively correlated with the basal area of Cryptomeria japonica. Each plot was colonized by any one species of Pterocarya rhoifolia, Phellodendron amurense, Swida controversa, Betula maximowicziana, Magnolia obovata, etc., or more. Among them, water-demanding species such as Pterocarya, Phellodendron, Swida, etc. tended to be distributed in the plots with lower basal area of Cryptomeria. This should reflect the abundant water in early spring resulting from the greater snow depth. Some plots experienced significant compositional shift during the 5-years study. This suggested that the communities of native trees within the plantations were still under the unstable composition.