Abstract: For the rock slope with planar failure surface, the structural plane plays a controlling role in the stability of slope. In the process of design and analysis, if the planar failure appears without fixed plane in rock slope, the critical inclination of failure face can be used as the basis for analyzing the slope stability. The traditional estimation formula is optimized. Considering the effects of gravity and earthquakes, two formulae for calculating the critical inclination of failure face are given respectively. One is based on the limit height of slope, and the other is based on the limit factor of safety. Through comparison and analysis, their advantages and disadvantages are obtained.