The ability to accurately and precisely measure the carbon stored and sequestered in forests is increasingly gaining global attention in recognition of the role forests have in the global carbon cycle, particularly with respect to mitigating carbondioxide emissions. Carbon stock was therefore estimated in Gambari, Osho and Shasha forest reserves in Southwestern Nigeria using forest inventory-based approach. Five 20 x 20 m sample plots each were laid in the Tectona grandis stands in the study locations making a total of fifteen plots. All the trees within a plot were measured for diameter at breast height (dbh), total height and merchantable height. The tree that had its dbh closest to the mean dbh was selected for destructive sampling to estimate its biomass. Four regression equations were developed out of which the one that had the best technical performance was selected to estimate biomass for all the trees measured. The mean plot biomass in each location was calculated and used to obtain biomass per hectare. Half of this value gave carbon per hectare value for each location. Carbon stocks per hectare were 14.84t, 29.36 t and 24.36 t in Gambari, Osho and Shasha respectively. Assuming the total areas of the reserves were covered by trees, it would mean large amount of Carbon would be stored and this would have great implications on carbon that would be sequestered from the environment. This would in turn mitigate the effect of CO₂ being released from other sources especially expansion of agricultural lands and fossil fuel burning.