An expert system that diagnoses the malfunction of local area network is developed. The system detects specific devices in the network as the source of thd deta disconnection. These soures are sct to goals in the knowledge base and rules are constructed by uncluding all possible occurrences un thd connection of therminals and host computers. An approach via OR graph is employed for thd systematic rule generation. The system is implemented in a shell and illustrative inference processes are presented.