Software reuse is to retrieve previously developed software artifacts from a repository according to given conditions. The retrieval is based on similarity measure. UML （Unified Modeling Language） class diagram is widely applied to software design， and its reuse as a core of software design reuse has attracted much attention. Therefore， the research on the similarity of UML class diagrams was carried out. UML class diagram contains semantic and structural contents. At present， the similarity research of UML class diagrams mainly focuses on semantics， and there are also some discussions on structural similarity， but the combination of semantics and structure has not been considered. Therefore， a hybrid similarity measure combining semantics and structure was proposed. Due to the non-formal nature of UML class diagram， the UML class diagram was transformed into a graph model for similarity measure， the Maximum Common Subgraph List （MCSL） was searched， a Maximum Common Subgraph （MCS） tree was created based on MCSL， and a hybrid similarity measure method was proposed based on MCS sequence. The semantic matching and structural matching were defined corresponding to concept and structure common subgraphs， respectively. The similarity comparison and similarity based classification quality comparison experiments were carried out， and the experimental results validate the advantages of the proposed method.