Quantifying the impact of smallholder tree plantation to better design prescriptions tailored to promote climate change mitigation is essential. The study was undertaken to assess the aboveground carbon dioxide sequestration potential of Acacia mangium. Fifty temporary circular sample plots were established comprising 3,910 trees. The aboveground carbon density of A. mangium Willd. was determined non destructively using Brown and Lugo formula, “AGB (ton per tree) = volume over bark (m3 per tree) * wood density (g cm-3) * biomass expansion factor (BEF). The study was able to develop a new biomass expansion factor equation for A. mangium Willd. The results of the study showed that as tree age increases, the tree biomass and carbon density also increases, but at slower rate from 3-17 yrs. The average aboveground biomass density of A. mangium at ages 3, 10 and 17 years were 3.33, 33.06 and 73.09 t ha-1, respectively. On the other hand, the mean aboveground biomass density increment at site indices 12, 15, 18, 21 and 24 were 4.04, 4.40, 4.99, 5.47 and 5.86 t ha-1, respectively. This study explicitly showed the great potential of smallholder A. mangium plantation in sequestering carbon dioxide.