We propose an approach for modeling in B the class operations which is up to now an open issue. Each class operation is mapped to a B operation. The operation and concerned data are mapped into the same B abstract machine. The calling-called dependency between class operations is used to allocate B operations of class operations in B abstract machines. The B implementation construct and imports primitive are used to model the calling-called dependency betwen class operations. The combination of our approach with rules for data mapping from UML notation into B notation, which are proposed elsewhere, give rise to a complete aproach for deriving B specification from an object oriented specification.