Visualization of object-oriented programs by class diagrams is a widely used technique. So far no commonly agreed aesthetic criteria have been recorded in order to standardize and measure the quality of class diagrams. In this paper we focus on UML class diagrams, the standard notation for class diagrams in software engineering. We propose some aesthetic criteria which reflect the highly sophisticated features of UML class diagrams, a layout algorithm which respects all these features and an implementation of a graph drawing framework which is able to produce drawings according to these criteria.