Assessment of understorey species of a tropical rainforest ecosystem in South-western Nigeria was undertaken in International Institute of Tropical Agriculture (IITA) forest reserve, Ibadan. A total of twenty-four permanent sample plots of 0.0625 ha were used for the assessment of understorey composition, density and frequency, Importance Value Index (IVI). Cluster Analysis by Hierachical Classification and Detrended Correspondence Analysis (DCA) statistics were computed using PAST software A total of 3,833 individuals of 128 species and 44 families were identified. Family Papilonaceae had the highest number of species (11) followed by Moraceae (10). Albizia zygia had the highest frequency of occurrence (24) and density of 169.33/ha. However Culcasia scandens had the highest density of 299.33/ha. The highest Importance value index of 13.82 was recorded for Culcasia scandens, followed by Chromolaena odorata (11.80). The least (IVI) 0.18 was recorded for Blepharis maderaspatensis, Carica papaya, Cissus pinnata. Similarity between paired plots varied from 0.16 to 0.75, Simpson diversity (0.95) and dominance of 0.05. The number of species present in each of the plots ranged from 0-39. Plot 84 had the highest species (39), high Eigen value (73.7%), length of ordination space (-2 to 6) and the location of all the plots in the first quadrant indicated that the environment was stable, indicative of minimal variation in floristic composition between plots and high heterogeneity of the site and species respectively. Cluster analysis by hierarchical classification of the identified plant species revealed 17 distinct groups of plant assemblages at 60-84%. Members of each plant assemblages suggest close similarities in ecological niche. These findings showed that the IITA forest is diverse in species composition and the diversity of the understory may act as a catalyst for successful natural forest succession. This may create a favourable environment for the establishment of native forest flora and habitat for fauna which ultimately enhance conservation of biological diversity. The study concludes that a proper protection from human interferenc and recommends scientifically sound management of undergrowth a to maintain and/or enhance biodiversity of the study area.