Assertion checking is a powerful tool to detect software faults during debugging, testing and maintenance. Although assertion documents the behavior of one component, it is hard to document relations and interactions among several objects since such assertion statements are spread across the modules. Therefore, we propose to modularize such assertion as an aspect in order to improve software maintainability. In this paper, taking Observer pattern as an example, we point out that some assertions tend to be croscutting, and propose a modulalization of such assertion with aspectoriented language. We show a limitation of traditional assertion and effectiveness of assertion aspect through the case study, and discuss various situations to which assertion aspects are applicable.