Abstract: In order to study the effect of suction on shear strength and dilatancy of an unsaturated expansive soil,suction-controlled direct shear tests were carried out on both the natural and re-compacted specimens.The soil was a silty clay with an intermediate plasticity and a medium expansion potential.It was clearly shown that the dilatancy of the expansive clay increased with the increase of applied suction for both the natural and compacted specimens.Matric suction contributed to the shear strength of the expansive clay via two different mechanisms: one was the contribution of capillary force on inter-particle normal stress,the other was the effect of suction on soil dilatancy.As a result of the second mechanism,the contribution of suction to the peak shear strength for the clay was more significant than that to post-failure shear strength,particularly at high suction range.As compared with the compacted specimens,the natural specimens were higher in the peak shear strength and the dilatancy due to the cementation effect of the iron and manganese oxides.The contribution of suction to shear strength for the compacted expansive clay was more significant than that for a compacted kaolin.