Abstract: A hybrid system consists of interacting discrete and continuous components. Usually, the continuous components are modeled by differential equations, while the discrete components are modeled by discrete event systems or automata. After the analysis of the micro structure of a hybrid system, a description language DDL is given for the design of hybrid systems. It can describe hybrid phenomenon naturally and precisely, describe design decisions conveniently, and, by delaying the combination of automata symbols and system denotations, bring about great flexibility for system design. Via the hiding of internal communications and system single step consistent product, the DDL description of a hybrid system can be transformed into a hybrid transition system.