Landslide is one of the major natural disturbances in Taiwan that restlessly characterizing landscape and persistently disturbs the society. The Chi-Shan River watershed experienced serious landslide damage following the extraordinary heavy rainfall (2748 mm in three days) brought by typhoon Morakot in 2009, and 12 subwatershed of the river was selected to evaluate the distribution of landslides. In this study, we evaluated the effect of pixel size on the accuracy of automatic landslide mapping to find out the optimal accurate and economical spatial resolution to monitor landslide dynamic. Landslide areas were identified through different spatial resolution images from the sensors already in existence (QuickBird, Formosa II and SPOT4) and resampled from Formosa II at 10, 20, 40, 60, 80 and 100m spatial resolution. The results of the analysis were verified using correlation analysis and T-test between Formosa II image and other pixel size. Furthermore, it will be an attempt to determine the optimal resolution for remote sensing based investigation of landslides area assessment in study area.