Temperature and precipitation are the most important climatic parameters that play an important role in behavior, distribution, development, survival and reproduction of insects, in addition to their impact on climate change and economic activity, especially agriculture. In this article, the situation of drought during recent years and its impact on outbreak of important pests of orchards and crops in Chaharmahal va Bakhtiari province were investigated. To study the drought phenomenon, SPI values (standard rainfall index) of different climatic stations were studied for 16-years from 1995-2010. To investigate the changes in the pest population, depending on the pest, different methods such as pheromone traps (leopard moth) or regular and weekly sampling of leaves (spider mites) were used. Results showed that three severe drought periods occurred in 1995, 2008 and 2010 in the province, the most severe of which was 2008. The drought conditions caused the outbreak of important pests, such as leopard moth on walnut and spider mites on bean and almonds in different parts of the province. Mean number of captured leopard moth during 2007, 2008 and 2009 were 75.5, 105.7 and 94.2 (in each trap/year), respectively which indicated increasing of the pest population in the dry year of 2008. Moreover, density of bean spider mite were 13, 22.5 and 18 mites on 2 cm2 of each leaf /week during the mentioned years. Proper understanding of climate change and pest population changes in a region makes it possible to adopt rational methods to manage the pest population and reduce the adverse effects of climate change