Probabilistic slope stability analyses have been adopted in study and geotechnical practice. But there are many misconceptions in the literature. One of these is the evaluation index of slope. The evaluation index will not always have the same meanings for the different slip surfaces. There are five kinds of slip surfaces in probabilistic slope stability analysis: slip surface of minimum factor of safety at mean parameters, mix slip surfaces of minimum factor of safety during each iteration, slip surface of minimum reliability index, slip surface of minimum factor of safety at specific parameters combination and slip surface of minimum factor of safety during each iteration in first order reliability method. For different slip surfaces the evaluation indices may be different. The relation among these evaluation indices is discussed and the applicability of the evaluation index is suggested.