Composed of many classes or modules, big software can be represented with network model. By extracting the topology of UML metamodel from the UML metamodel specification, the scale-free, small-world networks properties are revealed. Based on this observation, we come up with our algorithms that can classify all classes in UML metamodel into three kinds: core, general and leaf. Our algorithm can categorize all classes into several subgroups by three factors, i.e., degree, betweenness and weak link. It is illustrated through case study that this algorithm is effective at mining community structure in large software systems.