Seasonal and horizontal variation in microclimatic elements in closed forest and canopy gaps of two different sizes (200m2 and 140m2) were studied within a Shorea chinensis dominated tropical rainforest in Xishuangbanna. The following variables were measured: gross radiation, net radiation, light intensity, components of thermal balance, air relative humidity, vertical air temperature gradient, maximum and minimum air temperature, ground surface and leaf surface temperature. Variation of air, ground surface and leaf surface temperatures, air relative humidity and light intensity was greater in the large gap than in closed forest. Under dry season clear sky conditions both gross and net radiation in the center of the large gap were at least five times higher than in the closed forest ,and light intensity was at least ten times higher. Variation in air temperature and relative humidity in the large gap was also higher in the dry than in the rainy season. In the large gap center and in the closed forest about 70%～80% of the net radiation was used for evaporation. In the large gap there were two heating layers the seedling canopy and ground surface, while in the small gap only one heating layer was apparent the canopy of young trees. Heating intensity was also greater in the large gap. Diurnal variation in relative humidity was greater in the large gap than in the small gap.