Analysis of the carbon content in various organs of the sample rubber trees was taken by multiplying with the 0.5 conversion factor for the aboveground biomass based on the method reported by Atjay (1979). The mean carbon contents in the stems and branches of different age rubber trees were nearly the same (Table 2). They varied between 57.02- 57.61% (57.25%) for the stem, and 56.96- 57.62% (57.24%) for the branch. The mean content was 54.89% for the leaf and 57.08% for the root. As a result, the amounts of biomass carbon in the 1, 5, 10, 15, and 20-year-old plantations increased with the stand’s age;