Plant diversity was studied in the plantations of different species at Wondo Genet. The aim of the study was to evaluate the effect of overstorey species, age, and density of the different plantations on the understorey plant diversity. The plantations species were of Cupressus lusitanica, Eucalyptus grandis, Grevillea robusta and Juniperus excelsa. The age of the stands of the different species ranged between 6 and 23 years. A total of 72 plots, having 20 m x 20 m area were studied under the plantations. A total of 45 understorey plant species were recorded.There was a significant difference among the plantation stands with regards to understory plant density, diversity and number of species. The diversity and number of understory plant species were positively related with age of the plantation stands while understorey plant density was inversely related with overstory plant density and crown cover.The overstorey plantation appeared to affect the abundance of understorey plant species more than the species richness. Canopy cover was significantly associated with understorey plant density under all the plantation stands than other stand characteristics.