A study on the assessment of species diversity of large mammals was conducted in Chilalo Galema Mountain in east Arsi administrative zone from February 2019 to September 2019. The aim of this study was to assess large mammalian species diversity and human-wildlife conflict in Chilalo-Galema Mountain National Park. Line transects and observation methods were used to collect data. Twenty six transect lines were laid, varied in length from 1.41km to 4.34km. Transect width ranged from 200m to 400m. In this study both primary and secondary data were used. Direct observation, body parts, vocalization and indirect evidences such as footprints, burrowing and droppings were used to collect data on diversity of large mammals. Observation and direct count of animals were made during 6:00-10:00 a.m. in the morning and 3:00-6:00 p.m. in the late afternoon when most mammals were active. Species diversity of large mammals was calculated using the Shannon-Weaver index of diversity. The evenness of mammalian species was calculated as J=H’/H’max where H’max=ln(s). 28 species that belong to seven Orders (Primates, Artiodactyla, Carnivora, Hyracoidea, Tubulidentata, Rodentia and Lagomorpha) were identified and recorded during the study. Out of the 252 recorded mammals 26 (10.08%) were order primates, 50 (19.38%) were order carnivora, 1 (0.39%) were order hyracoidea, 6 (2.33%) were order tubulidentata, 164 (63.57%) were artiodactyla, 8 (3.10%) were rodentia and 2 (0.78%) were lagomorpha. Mammalian fauna of the Chilalo Galema Mountain was dominated by two groups of orders, the most abundant orders; namely, order artiodactyla and order carnivora, which contain 164 (63.57%) and 50 (19.38%) mammals respectively. The least abundant was order hyracoidean with 1 (0.39%) recorded mammal. So, Chilalo-Galema is rich in mammalian species diversity and needs conservation measures.