Soil respiration is a crucial carbon fiux for determining carbon balance in the forest ecosystem. In Kanchanaburi, western Thailand, we measured soil respiration at different stand ages (1, 6 and 21 year-old) of teak (Tectona grandis) plantations. There were no significant differences in soil respiration among plots of different ages. Soil respiration rates showed a clear seasonal pattern of a high rate in the rainy season from April to November and a low rate in the dry season from December to March. The contribution of organic layer, roots and soil to total soil respiration was 17, 15 and 68%, respectively, in the 6 year-old teak plantation (T-Y plot). Total soil respiration rate was significantly correlated with soil water content in the 0-30 cm layer. The annual amount of CO₂ effiux from the forest fioor was estimated to be 1,062-1,154 gC m^-2 y^-1 in the teak plantations in 1997. In 1998, annual CO₂ effiux declined to 80% of that in 1997 in the T-Y plot, probably due to low rainfall.