It has long been recognized that landslides largely take place along discrete shear zones, called slip zones. Microstructures of slip zones can therefore give direct evidence of conditions and mechanism of deformation leading to a landslide. In this study, microstructures of the natural slip zones of a landslide in granitic saprolite were comprehensively investigated using optical microscopy and electron microscopy techniques. Microstructural features of the slip zones were quantified using multiple spectrum image analysis method. It is found that geometric pattern of grain orientation in the slip zones are mainly controlled both by their particle size distributions and mineral compositions of grains. In the section of the slip zones dominated by coarse grains (>0.06mm), overall orientation of grains are random, but fine sand and silt size quartz grains appear to be aligned at an acute angle to the slip direction. Additionally, thickness of such slip zones seems to correlate with the abundance of quartz grains, and less with feldspar grains. When fine grains (<0.06mm) prevail in the slip zones, platy kaolinite and its aggregates display both parallel and inclined alignments. Thickness of this type of slip zones appear to be controlled by at least three factors: abundance of clay fraction, relative ratio abundance of kaolonite to halloysite, and thickness of pre-existing clayey weak zone. Consequently, the three types of deformation mechanism were postulated to lead and/or accompany shear, namely rotation, sliding and transition. Quantitative microstructural analyses have proven to substantially enhance the nature and quality of information obtained by visual methods.