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To clarify the sliding mechanism of a landslide, the shear behavior characteristics of slip zone soil are always vital. In the Northwest of China, thousands of landslides generally develop in three main types of slip zone soil, i.e., Quaternary loess, weathered Mesozoic-Cenozoic mudstone, and weathered Mesozoic-Paleozoic phyllite. In the present study, the shear behavior characteristics of these three kinds of slip zone soil were investigated through direct shear tests and their implications for the sliding mechanisms of landslide were also discussed. It was found that, in general, an abrupt failure of loess landslide could be related to the strong strain-softening behavior of loess in the direct shear test, while the ductile shear behavior of weathered mudstone and phyllite coincided with the creep deformation of the corresponding landslides. The residual friction angle of slip zone soil varied with the soil type and its mineral composition, and the lowest residual friction angle about 11.5° could be found in weathered phyllite (SET) with graphite. Compared with the previous research, it is interesting to find that the residual friction angles of mudstone and phyllite in the present study accorded reasonably with the empirical correlations proposed by Wen et al. (2007) for weak rocks in the Three Gorges area, China.