In order to study the effects of suction stress on undrained shear strength of unsaturated soil under low confining pressure, unconfined compression tests and constant water content shear tests, in which the suction value and the volume change were measured, were carried out using the compacted specimen of a silty soil. The calculated failure stress state parameters were then analyzed with the equivalent confining pressure by taking into account of three kinds of suction stress; one is obtained from the soil water retention curve corresponding to the degree of saturation and the measured suction, and the others are obtained from the soil water retention curve for the water content and suction value, s* decided by the suction stress - SWRC method (SSM). A unique failure line was obtained for both saturated and unsaturated cases. This verifies the relevance of new suction stress estimation method. In addition, the results also suggests that the unsaturated shear strength can be predicted from uniaxial compression test with specimens at different initial moisture contents if the SWRC for water content is known.