Software maintainability is one of important factors that developers should concern because two-thirds of a software system's lifetime-cost involve maintenance. Understandability is one of sub-characteristics that can describe software maintainability. Aspect-oriented programming (AOP) is an alternative software development paradigm that aims to increase understandability, adaptability, and reusability. It focuses on crosscutting concerns by introducing a modular unit, called "aspect". Based on the definition of understandability that "the related attributes of software components that users have to put their effort on to recognizing the logical concept of the components", this paper proposes seven metrics for evaluating understandability of aspect-oriented code using different levels of dependence graphs. The metrics are applied to two versions of aspect-oriented programs to give an illustration. © 2011 Springer-Verlag Berlin Heidelberg.