Combining object oriented model with value-based model would help getting the merits of both models. In this paper, based on NF² relational model with composited tuple identity, the operations of projection if and natural join keeping the nested structure of relations, are given. From this, the authors have studied the semantics and formallied definition for class, subclass, attribute inheritance in NF² relational model, introducing a special attribute R" and concept of S-subclass, discussed the common sub class and multiple inheritance. All of the discussions are based on composited tuple identity, possessing the features in O-O model.