A study was conducted from July 2015 to June 2017 in Hazarikhil Wildlife Sanctuary (HWS) of Bangladesh to find out mammalian species diversity, population density and identification of major threats to the mammalian species of this sanctuary. Various methods were used including transect survey to detect diurnal mammals, and camera trap to identify nocturnal mammals. A total of 33 species of mammals were recorded which belongs to 20 families under 9 orders. Among the recorded species, 13 species of carnivores, 4 bats, 2 Primates, and 6 species of rodents constitute the major pail of mammalian community. According to IUCN (2015), among the recorded 33 mammalian species, 11 species were threatened (4 critically endangered, 5 endangered, 2 vulnerable), 15 species were least concerned and 7 species were nearly threatened. Hunting and poaching, human-wildlife conflict, use of poison for fishing, road inside the forest, forest fire, encroachment for gardening, non-insulated electric wire, grazing were identified as major threats to the mammals of this sanctuary.