Abstract: The probability diagnosis idea is presented in this paper; A system model suitable for fault diagnosis purpose, which takes into account the dependencies between the different objects in the Local-Area Networks (LAN) is introduced. For fault diagnosis problem is a NP-C problem, a heuristic algorithm is developed. The algorithm overcomes the disadvantages of existing fault diagnosis algorithms like high computing complex and ignoring fault dependency. It is proved to be effective by our simulation results. And it can generate acceptable results with good performances.