The Forest areas of Dolok Sipirok Nature Reserve (DSNR) and Dolok Sibual-Buali Nature Reserve (DBNR) are part of Batang Toru forest landscape which is also the habitat of some protected and endangered wildlife such as Tapanuli orangutans (Pongo tapanuliensis). However, the forest has continued to experience widespread pressure and degradation or reduction, mainly due to forest conversion. Therefore, it is very notable to assess the composition of forest vegetation of DSNR and DBNR considering their very important role for the survival of the Tapanuli orangutan populations. The composition of forest vegetation in this research is known by vegetation analysis. The method vegetation analysis used is line transect. The length of the line transect is 1000 m which consists of 10 plots of each transect with a distance between plots of 100 m. Measurements are carried out on the growth rate of seedlings, saplings, poles and trees. Furthermore, the processing of vegetation analysis data is carried out to find out the Important Value Index (IVI), diversity index (H’), evenness (E) and richness (d) of vegetation species in the research area. The results showed that the composition of orangutan habitat vegetation in DSNR consisted of 107 species from 39 families, while DBNR forest consisted of 117 species from 45 families. Tree Syzygium sp1. species consistently have the highest IVI scores at all forest vegetation growth rates of both reserves. The diversity, evenness and richness of DBNR forest vegetation species tends to be higher than DSNR forest.