The aim of this study is to compare the amount of aboveground biomass (AGB) for different diameter classes between natural forest and a well maintained plantation forest. The comparison of AGB is conducted in two different ways. In the first method, the AGB is calculated as to per unit forest area (Mg/Ha). In the second one, the AGB is expressed in terms of per unit basal area of trees (Kg/cm2). From the first method the amount of AGB in natural forests and plantation forests are 248 Mg/ha and 294 Mg/ha respectively. AGB count in plantation forest is 18.54% higher than that of natural forest with the first method. For the second method, two species (Neem and Sal) were taken among all as they were ruling in number for both natural and plantation forests. In case when the second method was applied, the amount of AGB in natural and plantation forests for neem were calculated 0.31 kg/cm2 and 0.32 kg/cm2 respectively. For Sal AGB values for natural and plantation forests are 0.44 kg/cm2 and 0.47 kg/cm2, respectively. The study shows that for both the species when the diameter is lesser, the difference in AGB between two forests is more and in case of larger diameter, the difference between two forests is smaller. This happens due to silvicultural maintenance exists in plantation forest which is most important for younger trees. But for the natural forest, no silvicultural practices take place. © Springer Nature Singapore Pte Ltd. 2019.