A diagnostic technique that can estimate quantitatively wear amounts under lubricated condition was tentatively designed using the on-line particle concentration and counting technique. In this proposed device, the standing surface acoustic wave (SSAW) was used to concentrate the wear debris in the lubricating oil in a micro-channel, the debris were then delivered to an appropriately sized detector and scanned one by one. The size and number of wear debris in the circulating oil could be therefore measured and counted on-line. The volume of each wear particles was calculated by accumulating each volume in real time. The wear debris amounts obtained by this quantitative estimation technique were fairly similar to the measured values of mass loss of the specimens. This proposed device was conveniently embedded in the lubricating system. This study was expected to open new avenues for the real time fault diagnosis and life prognosis of machine.