Abstract: Model evolution involves a series of complex change activities. Based on model driven architecture, the relevant model evolution conceptions were formally defined to accurately describe model and model changes. In MDE, model changes can be classified as the primitive operations and composition operations. The primitive operations are identified as addition, deletion and modification operation and can be imposed on the model clement individually. Merge and union operations arc composition operations based on simple operations.

Key words: Model driven architecture, Model evolution, Model difference, Metamodel