Abstract: Some results obtained by several direct shear tests are introduced. Each test is conducted under three different stress levels, and for the same type of soil, the same test process with different water contents is taken, namely, let the soil samples under the condition of saturated water content and optimum water content, respectively. The results show that the matric suction of unsaturated soil acts as one kind of cohesion to strengthen the shear strength of the soil. At the same time, the suction has very little influence on the internal friction angle. The same soil sample is used in soil-water retention tests in order to obtain soil-water characteristic curve (SWCC) for drying and wetting process. From the corresponding characteristic curve, we can see that the soil strength is highly dependent on hydrological process applied in the tests. Several factors may strengthen the shear strength of the soil, and the matric suction is the most important one. Based on the experimental and analytical results, a modified but more intuitionistic form for Fredlund’s shear strength equation of unsaturated soil is proposed.