Abstract: A lot of facts from field investigations show that a time-dependent deformation of a high rock slope could occur before its instability. Accompanied with this process, a potential slIp surface develops gradually in the slope, and finally cut through due to progressive failure along it or some triggering factors. In this paper, the phenomena of time-dependent deformation of a slope on the left bank of Yalong River are described in detail. They reveal that the slope has experienced a long-term deformation and its mechanism can be considered as a slIp along the weak bed plane and buckling on the foot of the slope. Further evidence indicates that the deformation of the slope is quite light, since the slope is under a stable condition at present.