Abstract: This paper presents the interface shear properties between smooth and textured HDPE geomembranes (GMs) with geotextiles (GTs) by a large size direct shear test in dry and wet conditions, which are parts of the composite liner system of landfill. Experimental results show that the peak shear stresses occur at small displacements for smooth GMs with GTs while textured GMs with GTs need large displacements to reach the peak shear stresses. The strain softening behaviour is evident at the interfaces of both smooth GMs and textured GMs with GTs. The shear strength of textured GMs are higher than that of smooth GMs. Due to the suction effect between the GM-GT interfaces, in the wet condition displayed higher peak stresses than those corresponding to dry condition. The interface shear strength parameters are obtained by a straight line approximation. It is proposed that the cohesion parameter could be ignored in the landfill stability analysis if the interface in wet conditions.