In an open grid of services architecture, to reflect service reliability is a major security problem. In this paper a trust evaluation model based on compensation is presented to enhance the reliability and credibility of grid services by evaluating the services’ trust value dynamically. For evaluating services’ trust, the trust model combines consumers’ commendatory trust, services access frequency, service’s own assigned trust and the decrease of the trust. Both theoretical analysis and emulation experiments show that the model can make the service providers and service demanders voluntarily discard the intention of cheating and other malicious behaviors, and then promote the creditability of services, simplify the calculating process and reduce the cost of communication. It is a creative solution for identifying reliability of grid services.