Trapatability and activity of animals in a particular habitat will vary with time depending on various ecological factors. Here we mainly focused on effect of rainfall, environmental temperature and other environmental changes on mammal activity. Study was conducted in two habitats located in Hanthana mountain area: Pinus plantation and Natural secondary forest. Mammals were detected using hair tubes. Study was conducted five months from November - 2011 to March - 2012. Five small mammal species Mus musculus, Rattus rattus, Rattus norvegicus, Bandicota indica and Ratufa macroura were identified. Large number of hair samples was collected with high rainfall along with relatively high environmental temperature. Almost no hair samples were collected when high rainfall shared with low environmental temperature. As pinus plantation experienced forest fire, amount of trapped hair samples decreased. But following rainfall pinus ground vegetation reappeared and higher number of hair samples got trapped. These results suggest that small mammal activity is positively correlated with the environmental temperature and negatively correlated with the rainfall. It also shows that, small mammals prefer areas with high ground cover; vegetation or litter.