The state-of-art of unsaturated soils strength and the angle of internal friction with matric suction indicated by Fredlund are introduced; and the law of is analyzed. With the test results of plane strain constant stress ratio-unloading of undisturbed expansive soils
the angle of internal friction with matric suction calculated by Fredlund’s method is great than the effective angle of internal friction . The result is not accord with Fredlund’s theory; the reason is that micro- crannies were puffed during unloading test so sa to reduce the matric suction. More experiments analysis must be taken.