Knowledge of the faunal diversity, abundance and habitat preference are basics for the status determination and proposing appropriate conservation measures. A survey was conducted to assess the diversity, relative abundance and habitat association of medium and large-sized mammals of the Dati Wolel National Park, Western Ethiopia from September 2012 to April 2013. It was hypothesized that the area is rich in mammalian species that prefer to live in grassland and woodland close to water source. Four habitat types were assessed during the study (wetland, woodland, riverine forest and grassland). Line transect survey method was used to collect data in the four sampled habitats. A total of 28 mammalian species were recorded. Woodland was the habitat with the most diversity of mammals (H' =2.643) followed by riverine forest (H' = 1.60677), and the wetland was considered the habitat with the least diversity of species (H'= 1.04086). Hippopotamus amphibius (hippopotamus; 43.92%) and Syncerus caffer (African buffalo; 33.3%) were the most abundant species, while Mellivora capensis (honey badger) and Ichneumia albicauda (black backed jackal) were the least observed (0.099%, each). The greatest species similarity was recorded between woodland and grassland (SI=0.76) and the lowest was between woodland and wetland (SI= 0.4). Thorough inventory for faunal diversity, involving multiple seasons and all vertebrates, strengthening the now loose park management by involving the local people will ensure the sustainability of the ecosystem in supporting the riche biodiversity components.