Clay shale has been a problematic soil due to its weathering characteristic. Weathering is the degradation of shear strength due to exposure to water and air. As the depth of soil increases, the weathering effect would decrease, leading to different weathering grades. On a slope, the different weathering grades would lead to an interface slope failure. Until now, laboratory methods on finding the interface shear strength between the weathering grades seldom been investigated. This research will propose a method and give an evaluation of the results. An interface direct shear test has been conducted until its residual state between an undisturbed clay shale and weathering clay shale. The weathering condition of the bottom clay shale is achieved by soaking and drying the bottom of the sample using a two-days wetting-drying cycle. Results showed scattered values with different behavior for the soaked and unsoaked conditions for increasing weathering days. The soaked condition resulted in an increasing cohesion, increasing average stress ratio, and decreasing friction angle. The unsoaked condition resulted in decreasing cohesion, increasing friction angle, and increasing average stress ratio. Previous research shows that, the residual shear stress should have decreased as the weathering days increases. The cause for the contrary results is due to soil swelling and the absence of a separator when weathering the bottom clay shale. In conclusion, the test conducted is not quite suitable to understand the interface shearing behavior of weathering clay shale. Thus, a modification is suggested for future research according to the identified causes.