In the Grid, the reliability of the providers who manage the resources is usually unknown to the consumers. Unreliable providers usually fail to complete consumers? task. Therefore, consumers take the risk that they lose their payment for the resource access. Establishing reputation systems is an alternative to manage such risk. However, existing attempts to establish reputation systems in the Grid mainly focus on enabling reputation evaluation to measure the resource providers? reliability, but do not effectively address the inaccurate testimonies and malicious referrers, which generally deteriorate the performance of reputation systems. Against this background, this paper proposes a reputation system with the ability to mitigate the influence of the inaccurate testimonies and malicious referrers. Experimental studies show that the proposed system is efficient in mitigating the influence of these inaccurate testimonies and malicious referrers.