As the requirements of the systems change, it is desirable to take these changes into account. To do so, the software must be extended to consider the changes. But the problem is that the modifications on software artifacts may cause conflicts and inconsistencies in the structure of the software, which are difficult to find and time-consuming to correct. Hence, it is necessary to refactor software during its lifetime. In this paper, we present a formal approach to refactor the structure of the object-oriented models -specified through graph transformation systems-while preserving their semantics.