Abstract: The shear dilatancy has a significant effect on the seepage properties of rock joints, which is influenced by the roughness and normal stress acting on the rock joint surface. However, except for joint the surface roughness and normal stress, the infills also have an important effect on the shear dilatancy of infilled rock joints. So, in order to investigate the shear dilatancy of thinly-infilled rock joints, three groups of specimens which have the same surface roughness as Barton's standard profiles and different infill thicknesses are prepared firstly. Then the direct shear tests are performed on these specimens under three levels of normal stress, and the normal displacement vs. shear displacement curves and failure characteristics of thinly-infilled rock joints are obtained. The results show that the shear dilatancy is influenced by both the surface roughness and the infill thickness under low normal stress condition, whereas the surface roughness is the key factor influencing the shear dilatancy under middle and high levels of the normal stress. Furthermore, the failure modes of specimens include the shear and crush of infill particles and the shear of surface asperities, and the former failure mode becomes dominant with the increase of infill thickness. Then an empirical exponential expression is proposed to describe the peak shear dilatancy angle of thinly-infilled rock joints quantitatively by considering the weakening effect of infills on joint surface roughness, which is validated preliminarily by the direct shear test results and published data in literatures.