Although research on various diamond polishing techniques has been carried for years, some issues still need to be examined in order to facilitate application on large areas in a cost-efficient manner. A compositive technique for machining efficiently thick diamond films prepared by DC plasma arc jet is reported in the present paper. A two-stage polishing was applied on thick polycrystalline diamond films, by employing first electro-discharge machining (EDM) for rough polishing and subsequently mechanical polishing for finishing operations. Experimental results obtained clearly indicate the applicability of the proposed two-stage technique for fabricating transparent diamond films that can be used for the production of X-ray windows. Appropriate etching with EDM is an effective pretreatment method for enhancing the efficiency of rough polishing process in mechanical polishing of thick diamond film. The machined surfaces of diamond films are studied by Scanning Electron Microscope (SEM) and Raman Scattering Spectroscopy (Raman).