Passive Dynamic Walking is a technique that makes a robot walk naturally like a human adjusting to skews and slight slopes without active control or energy. It is energy efficient, but to set the initial condition for robots to walk stably is difficult. So when a Passive Dynamic Walking model is affected by a slip, it is not considered to walk stably. This paper analyzes how the gait stability of Passive Dynamic Walking robots is affected by a slip. The results of this study provide some ideas about how to apply such technique in a variety of environments or conditions.