With the integration of Web Services as a business solution in many enterprise applications, the Quality of Service (QoS) presented by Web Services is becoming the main concern of both service providers and clients. QoS requirements are recorded as part of the bi-lateral service-level agreement (SLA). In an open and service-oriented market, it is crucial to establish trustable SLA compliance monitoring and thus to assure clients the QoS claimed by providers are credible. In this paper, we propose a novel trustable mechanism to monitor and evaluate SLA compliance based on the Aspect Oriented Programming (AOP) paradigm. Authoritative monitoring aspects are supplied by a trustable SLA manager and by weaving the aspects into susceptible service runtime, service providers are ensured to monitor and report their service status obligatorily and accurately. Our mechanism is trustable, scalable and easy to deploy. We have engaged this mechanism into SOAPExpress and results show that it is effective.