Abstract: In variation-performance environments, multiple applications compete and share a limited amount of system resources, and suffer from variations in resource availability. QoS-aware applications are desired to adapt themselves to the dynamic environments for QoS guarantee in runtime. Since system resource is managed globally, adaptation mechanisms are built not only within an application, but also in a system where the application is running. In this paper, a QoS model (QuCOM) for component system is given to support application adaptation to dynamic system. For verification purpose, the results of an experiment example (a video stream application) are presented.