Abstract: The principle and performance of a newly developed on line ferrograph instrument is introduced. The instrument can automatically compute the composite ferrograph density and severity of wear index, and identify large diagnostic wear particles in lubricating oil, which can be used to judge the wear status and degree of a monitored machines with an oil cycling system. Alarms are also available given the machine operates under abnormal conditions. The comparative test results of the newly developed device and the conventional one indicate that the former can better meet the needs of on line wear monitoring of machines.