Cooperative game theory is a mathematical theory used for modeling negotiation problems between players. In these games, coalition formation is a central concept: for players, it?s the problem to be solved for getting a payment as best as they do. In this paper, we focus our attention on the dynamic of coalition formation. Our purpose is to determine which kind of rationality leads to an equilibrium, given a game theory model. A multi agent system paradigm is used to produce simulations. We?ve performed several simulations with different combinations of agents behaviors. We present how an agent prospects for a new coalition and how he accepts or not to be a member of a new coalition. Then, we display some situations in which simulation tends to an equilibrium inside the game core.