Service description usually presumes a representation of the world model. The Description Logic (DL) is an efficient way for representing the world model, esp. on Semantic Web, because of its framework, decidable reasoning, and popularity. DL can bring structure to services, but only DL itself is inadequate for modelling dynamic aspect of Web services. In this paper, Dynamic Description Logic (DDL) is proposed to combine DLs with action formalisms. The interaction between actions and the DL-based world model is embodied in two aspects. On one hand, DL knowledge base provides knowledge and information for the reasoning on actions; on the other hand, the information stored in DL knowledge base is changed by the execution of actions. In DDL, two basic reasoning tasks are defined to check precondition and effects of actions. Based on the relationship between DDL and a transition system, a reasoning support for DDL is also given by translating actions into logic programs. By the combination of DLs and actions, DDL brings a better view of how services impact the world, facilitates interoperation between services, and enables the reuse of already available algorithms and engines for service reasoning. Thus, it can provide a logical way for embracing actions into Semantic Web.