The sequence diagram was extended in the MARTE specification for modeling purpose, but it can not be used in the correctness verification stage. The OMG proposes to solve this problem by model transformation techniques; the model A is transformed to a formal model B which is equipped with efficient analysis or verification tools. ho describe the semantics of A by model B can guarantee the bi-simulation relation between them. A model named timed transition system for sequence diagram(TTS4SD)was proposed. At first,we offered the formal syntax of the sectuence diagram and the TTS4SD,then we described the semantics of the sequence diagram by I hS4SD. Taking the semantics as basis,the checking work was carried out on the TTS4SD.An example was given to describe the above process.