Prototyping is an efficient and effective way to understand and validate system requirements at the early stage of software development. In this paper, we present an approach for transforming UML system requirement models with OCL specifications into executable prototypes with the function of checking multiplicity and invariant constraints. Generally, a use case in UML can be described as a sequence of system operations. A system operation can be formally defined by a pair of preconditions and postconditions specified using OCL in the context of the conceptual class model. By analyzing the semantics of the preconditions and postconditions, the execution of the operation can be prototyped as a sequence of primitive actions which first check the precondition, and then enforce the postcondition by transferring the system from a pre-state to a post-state step by step. The primitive actions are basic manipulations of the system state (an object diagram), including find objects and links, create and remove objects and links, and check and set attribute values. Based on this approach, we have developed a tool of automatic prototype generation and analysis: AutoPA3.0. © Springer-Verlag London Limited 2008.