The objective of this study was to determine the stem density, biomass expansion and carbon stocks of Cryptomeria japonica species in the three stand age classes in southern part of South Korea. A total of 15 sample trees were harvested for data analysis. The results showed that the dry weight (kg/tree) of Cryptomeria japonica were 2.93 in stand 1, 168.72 in stand 2 and 402.27 in stand 3. The total aboveground biomass (ton/ha) for stand 1, 2 and 3 were 6.35, 184.74 and 251.35 while belowground biomass (ton/ha) in the three stands were 7.62, 227.77 and 308.14, respectively. It was also observed that as the stand age class increases, stem density values tended to slightly decrease and density observed ranged from 0.38 (g/cm3) to 0.34 (g/cm3). In addition, the above ground biomass expansion factors ranged from 3.65 to 1.28 and total biomass expansion factors ranged from 4.07 to 1.45 for the three stand age classes. This results showed that the above ground biomass expansion factor and total biomass expansion factor tend to decrease as the stand age increase. The influence of stand age on biomass expansion factor showed that it was statistically significant, while it was not significant on stem density