The diversity, distribution and relative abundance of medium and large-sized mammals were studied in the newly established Baroye Controlled Hunting Area, Southwestern Ethiopia. The study covers two seasons extending between January and September, 2014. Three habitats types, grassland, woodland, and riverine forest were identified in the study area. A line transect survey method was implemented to record the mammalian diversity. Of the 38 arbitrarily established transects, 11 (5 from the grassland and 3 each from the woodland and riverine habitats) were randomly selected and permanently surveyed for two seasons. Each transect was surveyed twice a month for three months per season. A total of 23 species of mammals from 7 orders and 13 families were recorded from the area. Only five species including the rock hyrax ( Procavia capensis ) and Bush hare ( Lepus fagani ) were from medium sized mammals. Among the 18 large sized mammals, the African buffalo (Syncerus caffer) was the most abundant. The leopard ( Panthera pardus ), lion ( P. leo ), African civet ( Civettictis civetta ), honey badger ( Mellivora capensis ) and black backed jackal ( Canis mesamolas ) were represented by few individuals. Riverine forest harbored the highest mammalian species (H’=2.37) followed by the woodland (H’=2.23) during the dry season. During the wet season, diversity was highest in the woodland (H’=2.25). The similarity of mammalian species between the riverine forest and woodland were very high during both seasons (SI=0.66), and least between riverine forest and grassland (SI=0.38). Owing to the high faunal diversities, upgrading the current controlled hunting area into higher protected area category will have immense significance to better conserve the unique ecosystem and faunal resource of the area.