In this paper, we propose a fine-grained coupling metrics suite for aspect-oriented (AO) systems, to measure software changes during system evolution. We also present a correlation model in terms of intermediate processes, for better evaluating the relation between coupling metrics and system maintainability. To investigate the practicability of our proposed model, we have implemented a coupling metrics analysis tool called AJMetrics and performed an empirical study on eight AspectJ benchmarks. The experiment result suggests that our correlation model provides useful information to evaluate the maintainability of AO systems.