QoS-aware　applications　can　satisfy　not　only　the　functional　requirements　of　the　customers,　but　also　the　QoS　requirements.　QoS-aware　Web　service　orchestration　translates　the　QoS　requirements　of　the　customers　into　those　of　its　component　Web　services.　In　a　system　viewpoint,　we　discuss　issues　on　QoS-aware　Web　service　orchestration　and　design　a　typical　QoS-aware　Web　service　orchestration　engine　called　QoS-WSOE.　More　importantly,　we　establish　a　formal　model　of　QoS-WSOE　based　on　actor　systems　theory.　Within　the　formal　model,　we　use　a　three-layered　pyramidal　structure　to　capture　the　requirements　of　the　customers　with　a　concept　named　QoS-aware　WSO　service,　characteristics　of　QoS-WSOE　with　a　concept　named　QoS-aware　WSO　system,　and　structures　and　behaviors　of　QoS-WSOE　with　a　concept　named　QoS-aware　WSO　behavior.　Conclusions　showing　that　a　system　with　QoS-aware　WSO　behavior　is　a　QoS-aware　WSO　system　and　further　can　provide　QoS-aware　WSO　Service　are　drawn.