The sun photometer was used to measure the diurnal atmospheric transmittance of autumn and winter in a mountainous area of South China. The results show that the transmittance of measurement location has distinctive characteristics of daily and monthly variation. The daily maximal transmittance appears around 12:00. The remaining bands show that the transmittance in October is greater than the other two months excluding water vapor band 936 nm. The band 936 nm has reverse characteristics. The vertical transmittance of the band 936 nm shows calmer variation than those of the remain bands whose vertical transmittance in October and November are obviously greater than the vertical transmittance in December. The vertical transmittance of the whole bands presents more variation before 10:00 and the results after 12:00 show calmer variation. The atmospheric transmittance or vertical transmittance of different months shows less fluctuation after 10:00, and the atmospheric transmittance or vertical transmittance of afternoon are currently greater than those of morning. The results can be used as references for the optical engineerings which are applied in the region.