The study investigated the composition and spatial distribution of the invasive aliens and native  species in the various vegetation physiognomies in Akure forest reserve, Ondo state, South west Nigeria. The forest reserve was demarcated into five study sites (A,B,C,D,E).  Site A-Natural forest, B-Teak plantation, C-Taungya system, D- Taungya +Teak +Gmelina, E- Teak +Gmelina + Pinus plantation. In each of dry and rainy seasons, ten study plots each measuring 25m×25m indicating two plots from each of the five physiognomies were selected for sampling. Within each plot, for all woody species that were{1m} and above, complete enumeration, separation of enumerated stems into aliens and natives, measurements of density, basal area, height, girth at breast height were determined. Shanon weiner diversity index of similarity, Sorensen’s index of similarity  were used to measure the degree of similarity among the sites in their species diversity and floristic composition Differences in species richness, Shannon weiner index and eveness between the most invaded and the least invaded physiognomies were used to measure the effects of invasion on the biodiversity. Student t -test was used to test for significant differences in basal area, stem density and size distribution between aliens and natives. Result showed that there were ninety three (93) woody species consisting of five (5) alien species and eighty eight(88) natives in all the study sites though the aliens had low densities. Alien species found in the study area were Tectona grandis, Gmelina arborea, Pinus  caribae, Gliricidia sepium and Carica papaya. The most invaded site which is a disturbed site being a source of revenue to the government is the teak plantation while the natural forest which is relatively undisturbed is the least with no alien species. These was no significant differences in the stem densities of the alien and native species. Size  distribution of all stems fall into the lower size classes. The least invaded sites had the highest basal area, species diversity, species eveness. Species richness while the most invaded site had the lowest in each case except the basal area. Other sites had intermediate values. The lowest similarities index of 5.55% was observed between the least and most invaded. It is concluded that invasive alien woody species are well distributed in Akure forest reserve though the natural forest is still intact and not vulnerable to invasion by alien species. Natural areas  resist plant invasion while disturbed areas are prone to invasion by alien plants which have significant impacts and effects  on the biodiversity of the forest reserve by reducing species richness, diversity and eveness.