In this paper, the author extends the concepts of functional dependency, multiple valued dependency, normal form and normalization in relational model to complex object model with nested structure. The semantics of entity relationship are captured in functional dependency and multiple valued dependency. The approaches and steps of normalization are proposed. In final normal form, there exists no any incomplete nesting, and on every level of nested structure there exists only the key with single attribute, which corresponds to the BCNF normal form in relational model. It could not only make redundancy minimum, avoid anomalies of update, but also improve query performance, guarantee to correctly and simply express entity relationship with nested structure and without flat entity relationship classes.