Abstract: The shear strength of unsaturated soils is composed of 3 parts:(1) the true cohesion, c;(2)the frictional strength due to external forces, (σ-u)tgφ; and (3) the internal friction produced by suction. The third component is called “suctional strength” and denoted by the symbol τ s in this paper. Although the shear strength theory of unsaturated soils has been established by Bishop and Fredlund in 1960 and 1978 respectively, it has rarely been adopted in engineering practice because of the difficulty in measuring suction and related parameters. It was discovered that the swelling pressure p s of unsaturated soils measured under constant volume condition might be in linear relationship with the suctional strength τ s. Some investigations in early 90′s further proved the above discovery and are reported is the paper. Since the measurement of swelling pressure is much more easier than that of suction, it may provide a new and convienent method of approach to this problem.