Abstract: An elastohydrodynamic lubricating film thickness measuring system, based on multi-beam interferometry, has been introduced in this paper. The measurement principle and the instrumentation were discussed. A simple and efficient counting method was suggested in order to obtain the fringe order of measured points. It is demonstrated that the present measuring system can provide continuous measurement for the lubricating film from nano to micrometers thickness with a resolution of nanometer levels, and is capable of investigating ultra thin EHL film and tiny variation of EHL film.