Abstract: One of the most advanced research problems in intrusion tolerance systems (ITS) is error detection, which has become another essential technique in system security to prevent the intrusion from generating a system failure. A parallel error detection method named PBL for ITS, which is based on distributed Bayesian learning, is proposed in this paper. This method is particularly suitable for detecting errors with distributed sources. The PBL method not only is useful in detecting errors in the distributed network environment, but also can be used to enhance noise tolerant ability of ITS. Some key problems of the PBL method in detail are also discussed.