Abstract: Red soils derived from granite in slope disintegration areas are influenced significantly by wetting and drying. Effects of wetting and drying changes on decay of soil shear strength in slope disintegration areas were investigated in laboratory by direct shearing test. Five soil wetting and drying treatments (drying 48 h, drying 24 h, natural water content, wetting 30 s, wetting 60 s) were tested. Experimental data indicated that shear strength indices (soil cohesive force and internal frictional angle) declined with drying and wetting levels, and they reached to the maximum at the water content of about 13%. The relationship between shear strength indices and drying and wetting levels could be simulated to linear function before strength peak and to first order exponential decay function after the peak; Shear strength was dominantly affected by crack evolution during drying period, while matric suction was the main factor for shear strength decline. Finally, the drying and wetting effect on development of slope disintegration was discussed in the paper.