Here, we used male Least Horseshoe Bats, Rhinolophus pusillus, to assess the change of colony size as well as the potential determinants throughout the summer. We used an infrared thermal imager to monitor the colony size of the bats inhabiting in a disused military fortification during June and August (Fig. 1). We applied one-factor analysis of variance (ANOVA) and the general linear regression model to test the monthly variation of colony size as well as to determinate relationship between bat colony size and microclimatic conditions in the cave. Analyses revealed that R. pusillus exhibited a marked increase in colony size from June to August (Fig. 2). The coefficient of variation in bat colony size was higher in June than that in July and August. The bat colony sizes were significant positive associations with ambient temperature, relative humidity in the cave (Fig. 3). These results highlight that microclimatic factors inside the cave affect the colony size of R. pusillus. Our findings provide a theoretical basis for roost conservation and management in wild bats.