Abstract: The shearing status for the landfill liner system located on the slope is different from that located on the bottom floor. A large-diameter inclined direct shear apparatus is designed to simulate liner shearing on various slopes. The comparative tests between the inclined and traditional direct shear apparatuses are conducted using the standard sand and the selected clay. It is found that both the displacement behavior and the shear strength of the standard sand and the selected clay obtained from the inclined and traditional direct shear tests are basically the same. More inclined shear tests are conducted for a composite liner interface between smooth HDPE geomembrane and compacted clay. All curves of the ratio of shear to normal stresses vs. the displacement under various vertical pressures show strain-softening behavior in the inclined shear test. The advantage of the inclined shear test is that more detailed shearing characteristics regarding changes of the normal and shear stresses on the shearing plane and the ratio of the shear to normal stresses can be investigated.