Abstract: Three representative forest gaps with pit-mound microsites formed by uprooted trees were selected within the 2.55 hm²  plot in a Pinus koraiensisdominated broadleaved mixed forest in Xiao Xing’an Mountains of Northeast China. The cleared land and closed stand were set up as the controls, and the PAR, air temperature and relative humidity in the centers of different size gaps and in mound top as well as the total radiation and precipitation in the gap centers were measured between July and September, 2011 by using multichannel automatic meteorological stations. The differences of the microclimate between the gap centers and mound top in different months were compared, and the monthly and diurnal variations of the microclimatic factors in the gap centers and in the mound top under typical weather conditions were analyzed. The results showed that the mean monthly PAR and air temperature in the three gaps of different sizes were in the order of large gap > medium gap > small gap, and the mean monthly relative humidity was in the order of small gap > medium gap > large gap. For the same size gap, the mean monthly PAR and air temperature were higher in the mound top than in the gap center, whereas the mean monthly relative humidity was higher in the gap center than in the mound top. Both the mean monthly total radiation and the mean monthly air temperature in the forest gaps and in the controls were in the order of July > August > September and of cleared land > large gap > medium gap > small gap > closed stand, while the mean monthly relative humidity was in the order of closed stand > small gap > medium gap > large gap > cleared land. The differences in the mean monthly relative humidity between closed stand and various gaps and between closed stand and cleared land reached significant level. The monthly precipitation from July to September decreased in the order of cleared land > large gap > medium gap > small gap > closed stand. Whether in sunny days or in overcast days, the mean daily PAR and air temperature were higher in mound top than in gap center, and the mean daily relative humidity was in opposite. Whether in mound top or in gap center, the mean daily PAR and air temperature were higher in sunny days than in overcast days, while the mean daily relative humidity was higher in overcast days than in sunny days.