Abstract: It is evident that unsaturated soils are usually encountered in nature and engineering practice. Hence, the understanding of unsaturated shear strength behavior is necessary. The shear strength behavior of unsaturated soils is investigated on the basis of pressure plate test and direct shear test. The results show that the angle of internal friction decreases with increasing degree of saturation and the cohesion attains its peak value at degree of saturation 40 % ? 60 %. According to these results, the unsaturated shear strength behaviors of non-cohesive and cohesive soils are demonstrated. The internal friction angle of non-cohesive soils increases with increasing matric suction; and the pseudo-cohesion attains its peak value in primary transition zone and then decreases. The cohesion of unsaturated cohesive soils increases with increasing matric suction until threshold suction reaches. The threshold suction is the minimum suction at the residual zone of unsaturation. The cohesion of unsaturated cohesive soils can be predicted before the threshold suction, because value is approximately constant. Moreover, the state paths influence cohesion in excess of the threshold suction.

Key words: unsaturated soils, shear strength, matric suction, soil-water characteristic curve