Most of the time, the construction of legged robots is made in an empirical way and the optimization of the mechanical structure is seldom taken into account. In order to avoid spending time and money on the construction of many prototypes to test their performance, a CAD tool and a methodology seem to be necessary. In this way it will be possible to optimize on one hand the kinematic structure of the legs, on the other hand the gaits which will be used by the future robot. Thus, we have developed a methodology to design walking structures such as quadrupeds and bipeds, to simulate their dynamic behavior and analyse their performances. The feet/ground interaction is one of the major problem in the context of dynamic simulation for walking devices. Thus, we focus here about the phenomenon of contact. This paper describes a general model for dynamic simulation of contacts between a walking robot and ground. This model considers a force distribution and uses an analytical form for each force depending only on the known state of the robot system. The simulation includes all phenomena that may occur during the locomotion cycle: impact, transition from impact to contact, contact during support with static friction, transition from static to sliding friction, sliding friction and transition from sliding to static friction. Some examples are presented to show the use of this contact model for the simulation of the foot-ground interaction during a walking gait.