Abstract. There are only very few approaches to normalizing objectoriented data. In this paper we present an approach to normalization of the object-oriented conceptual model based on UML class diagrams. First part of the paper describes the current status in the area of formal methods used for object-oriented data modeling. Second part presents four normalization rules, which are based on own experience and modi ed Ambler-Beck approach. These normalization rules are introduced on an example. Our method has been used in education at several universities. It has been and is also used for database design in software development projects, which we carried out. Recently, development of the CASE tool based on this approach has been started. Keywords. Data normalization, object-oriented data model (ODM), relational data model (RDM), rst object-oriented normal form (1ONF), second objectoriented normal form (2ONF), third object-oriented normal form (3ONF), fourth object-oriented normal form (4ONF).