Abstract: In order to verify the suction constitution in unsaturated soils, some un-hygroscopic tests are designed and performed. The testes are classified into four different types including common triaxial tests, pressure plate & common triaxial tests, pressure plate & GDS triaxial tests and natural air drying & common triaxial tests. The results indicate that the friction angle related to unsaturation can be greater than that induced by net normal stress in a higher saturation state and the strength increment induced by unsaturation is linear to matric suction in a lower saturation state. Failure occurs distinctly in a slope for unsaturated soil samples but this is not true for the saturated one.