In this paper an attempt has been made to develop an expert system called VIBMASTER to help the plant maintenance persons in diagnosing the cause of excessive vibration at the bearing supports of a rotating machinery. In the expert system a data base has been compiled from Standards, Journals, Handbooks and Rules from Maintenance Manuals related to maintenance engineering and management. The transfer of knowledge from these sources into the expert system for diagnosis is achieved with knowledge engineering procedures using if and then rules. The developed software tool can identify the fault(s) using vibration characteristics like velocity, speed of machine and signatures. The proposed system has been developed on Microsoft Windows environment and is written in Microsoft Visual Basic and Visual C++. To validate, the expert system is tested with the data collected at the bearing supports of a Boiler Feed Pump of a Boiler Feed Pump train of a large utility thermal power plant.__ampersandsignnbsp;