Abstract: The thick and flat multi-layer diamond films were prepared by hot filament chemical vapor deposition（HFCVD）for high frequency surface acoustic wave devices.The diamond grain size reduces gradually by changing deposition parameters and the surface grain size decreases approximately to 20nm.The total thickness and surface roughness of the multi-layer film were 35.81μm and 18.8nm（2μm×2μm） evaluated by field emission scanning electron microscopy（FE-SEM） and atom force microscopy（AFM）,respectively.These results show this multi-layer diamond film is fit for the substrate of high frequency surface acoustic wave devices.