In this paper, the major ideas of the normalization theory for object-oriented data models proposed by Tari et al. is introduced, their methods for creating an object normal form are analyzed, and the problems in these methods are pointed out. In order to develop a new method for creating such an object normal form, the meaning of a vertex of the directed graph is extended, such that it is not only a simple vertex, but also a directed graph. Based on this extended directed graph, an algorithm for creating it is proposed, the time complexity analysis of the algorithm is given, and its correctness is proved.