Aspect-Oriented Programming enables modular implementation of crosscutting concerns, but puts limitations on modular reasoning in software systems. Modular reasoning is the process of understanding a system one module at a time. In an AOP system, a part of module’s behavior can be defined or extended separately from the module, but that module has no knowledge of this extension. Therefore, decisions about a module can only be determined by reasoning globally. We propose an approach to help reasoning in AOP become modular. Tree Aspect-Aware Interface is an interface which contains infomation about modules of core concerns and crosscutting concerns, and it shows how they interact with each other to form a complete system. We introduce a case study to demonstrate how the information on tree aspect-aware interface is useful for modular reasoning in AOP, and a tool to facilitate our approach.