In wearable computing environments, a user brings and uses his/her own computer to acquire various services wherever he/she goes. This type of computer, which acts as a service platform for wearable computing, needs autonomy and simplicity of services, flexibility of services/devices configuration, and power saving functions. Since most conventional wearable computing systems do not fulfill all of these requirements, we propose A-WEAR, which is a rule-based wearable computing system. We employ an event-driven rule as a behavior description language of A-WEAR to achieve autonomy and simplicity. Furthermore, we employ a plug-in mechanism to achieve flexibility and power saving. Using our system, we can easily provide and use various services for wearable computing environments.