Abstract: In this paper, the concept of generalized degree of freedom is proposed according to the translation equations of displacements of compatible isoparametric elements. The correlation between compatible and incompatible isoparametric elements was proved by applying some restrictions to the generalized degrees of freedom. Thus, it is elucidated that the essence of inner degrees of freedom of incompatible elements is the degeneration of generalized degrees of freedom, and the incompatible elements are the special case of compatible elements.