Abstract: Shear strength of rock discontinuities is an important parameter in landslide prevention . It is necessary to study the effects of the two shear strength parameters, i.e., the cohesion c and the internal friction angle , in order to choose a suitable method for determining the shear strength parameter values of rock discontinuities. This paper focuses on the effects of the shear strength parameters of control discontinuities on the stability of rock slopes from 10 to 30 m high, where the limit equilibrium theory is used. The results show that the effect of the cohesion reduces with increasing in slope height and the effect of the internal friction angle increases with increasing in slope height. A group of curves are presented. The effect of the cohesion and the effect of the internal friction angle are equivalent on the curves. The effects of the cohesion c and internal friction angle on slope stability can be easily analyzied with the aid of the curves.