In this study, leaf area index (LAI), light transmission, and gap were estimated by using the hemispherical photograph analysis technique with Hemisfer software version 1.5.3 for a temperate mixed deciduous forest. The height of the overall vegetation in the stand ranged from 15 to 20 m, and the diameters of the trees in the stand at breast height varied between 14 and 28 cm. This study showed that the mean value of LAI increased by 4% in the 5-year period from 2007 to 2012. LAI was significantly (P < 0.05) greater in 2012 than in 2007, whereas the mean percentage of light transmission was significantly lower in 2012 than it was in 2007. In addition, there was a significant (P < 0.01) correlation between LAI and light transmission, and there was also a statistically significant correlation between the gap and light transmission (P < 0.05). According to long-term meteorological data (1982-2012), this study also demonstrated that the mean air temperature rose approximately 1.0 degrees C between 2007 and 2012. Consequently, this study indicated that LAI varies with respect to time and the increase of LAI leads to a decrease in light transmission. Additionally, the air temperature and precipitation have a significant influence on the LAI.