We estimated biomass and carbon stock from allometric relationships between tree size parameters (diameter at breast height [DBH], tree height) and plant part biomass (leaves, stems, total root biomass) in young teak (Tectona grandis L.f.) plantations of different ages (5–33 years) and with different site　index values in Thailand. In total, 101 trees from 18 sites and 76 trees from 15 sites (stand age: 5–33 years) were harvested to estimate above-ground biomass (stems, branches, leaves), and below-ground biomass. The coefficients of correlation for the obtained allometric relationships between tree DBH and stem,　branch, and aboveground biomass showed high values, ranging from 0.94 to 0.99 at each site. DBH also had a high correlation with below-ground biomass (R²=0.90). Similar wood density of the sampled trees might have reduced site-specific differences. The present results　indicated that stem, branch, above-ground biomass, and below-ground biomass might be estimated from our allometric equations with DBH data even when collected from site with different index values, and climatic conditions. Stand-level carbon stock amount was estimated using the present equations. Above-ground and below-ground carbon stock ranged from 1.3 to 67.7 Mg ha^-1 and from 0.4 to 13.7 Mg ha^-1, respectively. These values were similar to other studies in Thailand.