Forest stands have temperature retain abilities after sunlight radiation going down, those abilities are strong correlation with forest structure. In this investigation, the nature hardwood stand has a higher ability than Chain-fir (Cunninghamia lanceolata) plantation. It is because the LAI and relative light intensity in stands of hardwoods are higher than that of fir plantation. There is a significant correlation between cumulative temperature and CO_2 concentration in night time (18:00~06:00). In summer time of July has a higher cumulative temperature and has higher CO_2 concentration than winter time of January. The CO_2 concentrations in stand canopies of these two forests are higher than open air of outside stand. These phenomena indicated that there has an ability to retain temperature in forest canopy.