Abstract: To study the effects of cyclic stress ratio and principle stress axis rotation on the undrained cyclic response characteristics of K₀-consolidated stiff clay whose K₀ is larger than 1, a series of traffic loading tests with a large number of cycles are conducted on the intact Gault and Kimmeridge clays by using the dynamic triaxial test system (DYNTTS) and the hollow cylinder apparatus (HCA). The test results indicate that the stiff clay has strong brittleness compared with the soft clay. With the increasing cyclic stress ratio, the cyclic deformation responses can be categorized into stable, metastable, and unstable patterns. Under the continuous rotation of the principle stress axis, the axial strain of the stiff clay is gradually accumulated at the extension side, and its resilient modulus is rapidly degraded.