As a de facto standard for system modeling, UML diagrams have been widely used in the industry. Of these, the class diagram is useful to visualize the classes and their relationships and thus directly influences the forward and reverse processes of system modeling and development. Many earlier studies have attempted to examine as to what impact the layout and complexity of the class diagram would have on the comprehension of system modeling. However, their findings have been equivocal and it is not easy to understand the effects of diagram layout. Accordingly this research relied on the guidelines of diagram layout suggested in the earlier studies and designed a 2 (layout) x 2 (complexity) factorial design to examine their impact on diagram comprehension. 47 subjects were participated in the experiment where class diagrams were provided differently in their layout and complexity. We found that a good layout was significantly more effective than the bad. Furthermore the results were significantly persistent in the complex group than the simple. However, there was no interaction effect between layout and complexity of the diagram.