Abstract: Based on dynamic description logic, a model of mental states for an agent is presented The representation, reasoning and revision of mental states are investigated This model makes good use of the uniform formal framework of dynamic description logic, and describes the mental states of an agent in two aspects: static knowledge representation and reasoning as well as dynamic running and changing The problems of representation, reasoning and revising for agent mental states are explored Additionally, this model embodies the essential characteristics and running mechanism of an intelligent agent and provides solid foundation for the design and programming of an intelligent agent