Model integration is an important section of the model management research area, which can be divided into two parts: one is the integration based on model definition, the other is the model manipulation based integration. The primary foundation of the model integration based on model manipulation is the relationship of input and output between models. The paper puts forward a formalization representation of model, and presents some concepts, such as the combination model relation, the composite model and so on. Additionally, the existence of the model integration is also analyzed in detail and several sufficient conditions are proved.