Nkoanenkole Forest Reserve (NCFR) is such a potential area in floristic diversity within a relatively small area of 370.2 ha. The study was conducted in 2018 to analyze the floristic diversity and quantify the tree species aboveground biomass and carbon stocks of Nkoanenkole Catchment Forest Reserve (NCFR) in Arumeru District, Arusha-Tanzania. Systematic random sampling was applied, whereby a 24 with a 15 m radius (706.5 m2) were set at an inter plot and transect distance of 350 m. The plant species were identified by their botanical names, determined for abundance, and trees with ≥5cm DBH were measured. Shannon Wiever (H’), and Simpson indices were applied to calculate the index of diversity. Also, density and relative density were calculated. The AGB and AGC mg ha-1 were calculated using allometric models. 147 plant species were identified from the 24 sample plots, of which 54 were trees, 31 shrubs and lianas, 48 herbs, 3 sedges, 6 grass species, and 6 belonged to the ferns. The calculate H’ was 3.3316 for trees with an index of dominance of 0.053, and 2.449 for shrubs and lianas, sedges, herbs, and ferns with an index of dominance of 0.00.3135. The families of Euphorbiaceae, Rubiaceae, Fabaceae, and Asteraceae were determined to be the most dominant in terms of the number species. The identified tree seedlings were 14 with an abundance of 136, the density of 16.513±0.590, and a relative density of 20.14±0.73. Bersama abyssinica and Psychotria riparia were the most dominant tree seedlings. The DBH-class of 15±5 had the highest AGB (kg) and AGC mg ha-1, followed by 25±15, 55±45, 65±55, and the rest. NCFR is latent for its biodiversity, hence recommending further study on biological diversity, the possibility for upgrading the NCFR to the nature reserve, encourage beekeeping as potential pollinators, discourage illegal human activities through education and regular patrols.