Abstract: Three gaps with different sizes （larger：. gap 1,middle： gap 2 and small：gap 3） were selected within secondary forest ecosystems in eastern montane regions of Liaoning Province,China.Temporal variation in photosynthetic photon flux density （PPFD）,air temperature and humidity,soil temperature and soil water content at top layer （0-15cm） and sub layer （15-30cm） were measured in initial growth period. Results indicated that the maximum at center points of PPFD at the daily scale in three gaps eceurred at different time. The highest PPFD was at centers in three gaps, and the average PPFD at northern points is higher than that of southern points. Differences in gap size and ,sithin-gap position produced dramatic effects on direct, indirect and global site factors in this study （P〈0.05）, and global site factors was significantly correlated with average PPFD （F〈0.O1）. The soil temperature in gap 1 and 2 have positive correlation with their PPFD.The soil water content at top layer were significantly influenced by gap size （P〈0.05）, but that at sub layer.The larger the gap was, the higher the air humidity became. The above mentioned results suggested that the differences in gap size and within-gap position result in the heterogeneity of microclimate,and have an important effect on seed germination,seedling growth and development,as well as species distribution, which would influence forest regeneration and succession finally.