The permanent sample plot in Onigambari rainforest reserve, South-western Nigeria was enumerated for tree species composition and diversity. Systematic cluster sampling technique was adopted to allocate sample plots. 200m × 500m area and partitioned into 200m × 200m tracts. The tracts were 100m apart. The clusters were located within the reserve where human interference on the vegetation is relatively low. In each of the sample plots tree species ≥ 10 cm DBH were enumerated and recorded. The total number of 136 stands distributed among 35 species and eighteen families. From the result obtained, Triplochiton scleroxylon had the highest RIV (26.47), which is followed by Strombosia postulata (11.03), Cedrela odorata (4.41), Cola millenii (4.41) and Funtumia elastic (4.41). The dendogram of the study plots revealed two major clusters which consists majorly of Triplochiton scleroxylon in a cluster and Strombosia postulata and other species in the second cluster. Dominance index value of 0.09, Simpson index of 0.9, Shannon index of 2.96 and Evenness index 0.55 were obtained from the diversity indices. The species richness was high while diversity index was moderate. Deforestation and other anthropogenic activities should be greatly curtailed to prevent further loss of biodiversity so that the forest reserve can perform other ecosystem functions.

Key Words: Species, Diversity, Cluster, Evenness, Dominance, Biodiversity