Planar sliding is one of the frequently observed types of failure in rock slopes. Kinematic analysis is a classic and widely used method to examine the potential failure modes in rock masses. The accuracy of planar sliding kinematic analysis is significantly influenced by the value assigned to the lateral limit angle γlim. However, the assignment of γlim is currently used generally based on an empirical criterion. This study aims to propose an approach for determining the value of γlim in deterministic and probabilistic kinematic planar sliding analysis. A new perspective is presented to reveal that γlim essentially influences the probability of forming a potential planar sliding block. The procedure to calculate this probability is introduced using the block theory method. It is found that the probability is correlated with the number of discontinuity sets presented in rock masses. Thus, different values of γlim for rock masses with different sets of discontinuities are recommended in both probabilistic and deterministic planar sliding kinematic analyses; whereas a fixed value of γlim is commonly assigned to different types of rock masses in traditional method. Finally, an engineering case was used to compare the proposed and traditional kinematic analysis methods. The error rates of the traditional method vary from 45% to 119%, while that of the proposed method ranges between 1% and 17%. Therefore, it is likely that the proposed method is superior to the traditional one.