This paper proposes a graph transformation based description language of model refactoring (GraTDeLMoR) to formalize model refactorings. It designs basic elements of the language according to the features of model refactorings, and proposes approaches to describe model refactorings with these basic elements. It also proposes steps of the application of refactorings formalized with the proposed language, and provides corresponding CASE support. The paper discusses the descriptive ability of the description language with some typical examples, and results suggest that the proposed language is expressive in formalizing model refactorings.