Model merging plays a chief role in many software engineering activities, e.g. evolving Unified Modelling Language (UML) models for adding new features. Software developers may evolve UML models using merge relationships. However, given the growing heterogeneity of merge strategies and the UML limitations for expressing merge relationship, it is particularly challenging for them to specify merge relationships. Consequently, developers, end up expressing improperly merge relationships. Today, the UML can neither specify the semantics nor the order in which merge relationships must be performed. Developers are unable to specify the semantics and order in which merge relationships must be performed. This study, therefore, proposes UML2Merge, which is a UML extension for expressing merge relationship. The UML2Merge was evaluated through an empirical study with 10 participants for investigating its effects on the merge effort, the correctness of merge relationships, and the participant’s acceptance. The collected data suggest that the UML2Merge is proper to express merge relationships by requiring a low merge effort, producing elevated correctness of merge relationships, and having a high acceptance of the participants. The results are encouraging and show the potential for using UML2Merge to express the evolution of UML models through merge relationships.