At the core of MDD (model driven development) are the concepts of model and its transformation and refinement. Unified modeling language (UML) is selected by object management group (OMG) as a standard modeling language and model driven architecture (MDA) is constructed on it. However, UML models are not precisely described, especially in semantics. Thus the models developed in different phases or constructed in different views are not easily to be integrated together in MDA. In this paper, based on Hoare and He’s unifying theories of programming (UTP), a method is proposed to combine the refinement calculus of object systems (rCOS) with UML is proposed to increase the precision and transformation ability of the models. Models at different abstract levels and different views are constructed and integrated together to form a unified modeling system.