This study assessed the effect of the change in forest structure including understory vegetation of Sasa nipponica on soil C and N accumulation from 0 to 40-yr Japanese oak ( Quercus crispula ) plantation chronosequence. The peak of understory biomass was at 7 years after afforestation. Biomass of the overstory species linearly increased, and the biomass of overstory species was more than that of understory species after 8-yr. According to leaf mass index change along with stand age, canopy closure was occurred about 12 years after afforestation. The ranges of temporal changes in both soil C and N accumulations increased from surface to deep soil layers. The changes in C and N accumulation of O layer were large during early stage of stand development, and C and N accumulations increased until 10 years and 20 years after afforestation, respectively. Overstory leaf mass positively correlated with C and N accumulations in O layer. Understory biomass negatively correlated with N accumulation in O layer and positively correlated with C/N ratio. The N accumulations both in mineral soil 0-4 and 4-8cm in depth were negatively correlated with fine root mass of S. nipponica . The results suggested that change in vegetation structure such as canopy closure by overstory and dynamics of understory biomass affected soil C and N accumulations.