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MAC (mandatory access control) model is a basic security model for describing and verifying information system. In this paper a method is presented to verify security policies in MAC model by means of colored Petri nets (CPN). The main theme of this paper is depicted as follows: firstly, based on the lattice model of multilevel security (MLS) and Bell-LaPadula model, the MAC model is formally defined. Subsequently, an equivalent MAC model described by colored Petri nets (CPN) is proposed. According to the derived state reachability graph, four security properties of access control policy relations in MAC model are investigated, i.e. access temporal relations, access reachability, covert channel analysis, inference of sensitive information. An example of the security model is given for illustration. The results show that this concise graphic analysis method is suitable for formal verification. This method can be efficiently used to analyze information flow security and therefore improve the whole access control policies during security design and implementation of the system.