In order to evaluate the characteristic of shear behavior for unsaturated soil under the low confining pressure, a series of the unconfined compression test and the constant water content compression test were carried out using compacted silty clay. This study then examined the effect of the compaction energy according to the variation of the number of compaction times. The negative pore water pressure and the volume change of specimen during shearing were measured under the undrained condition. The peak strength, the post peak strength and the deformation behavior were also examined. As a result, it was found that the post peak strength states are arranged on the failure line of saturated state by considering the suction stress (ps) as a stress component in (pnet+ps, q) space regardless of the compaction energy. The state boundary surface could also be observed in the relationship between the stress ratio, (q/(pnet+ps)) and the specific volume. Therefore, these results can be interpreted that the critical state for unsaturated soil exists in the unconfined compression test under the low confining pressure.