The control problem of hybrid systems have received considerable attention. However, because of the existence of constraints and the combinatorial nature of continuous time and discrete event dynamics, the understanding of hybrid systems is rather limited at present. Only optimal control approaches were proposed based on heuristic rules. Few theoretical properties of system can be predicted until now. In this paper, we consider the tracking control problem of hybrid plants represented by MLD model to follow a family of reference signals produced by an external generator. Some new results are presented. The internal model principle of continuous system is extended to hybrid systems so as to solve the problem.