This study was conducted to develop allometric equations and to determine the stem density and biomass expansion factor (BEF) for the estimation of the aboveground and belowground biomass of Cryptomeria japonica in Jeju Island, Korea. A total of 18 trees were harvested from the 40-year-old C. japonica stands in Hannam experimental forest, Jeju Island. The mean biomass of the C. japonica was $50.4Mg;ha^{-1}$ in stem wood, $23.1Mg;ha^{-1}$ in root, $9.6Mg;ha^{-1}$ in branch, $4.6Mg;ha^{-1}$ in needle and $4.3Mg;ha^{-1}$ in stem bark. The diameter at breast height (DBH) was selected as independent variable for the development of allometric equations. To evaluate the performance of these equations, coefficient of determination ($R^2$) and root mean square error (RMSE) were used and results of the evaluation showed that $R^2$ ranged from 71% (root biomass equation) to 96% (aboveground biomass equation) and the RMSE ranged from 0.10 (aboveground biomass equation) to 0.33 (root biomass equation). The mean stem density of C. japonica was $0.37g;cm^{-3}$ and the mean aboveground BEF was $1.28g;g^{-1}$. Furthermore, the ratio of the root biomass to aboveground biomass was 0.32.