A newly developed nano/microcrystalline diamond composite film for thermal applications was prepared in this investigation. A microcrystalline diamond (MCD) film was deposited onto silicon substrate by hot filament chemical vapor deposition (HFCVD) method, and then a nanocrystalline diamond (NCD) film was grown onto this MCD film to obtain a NCD/MCD composite film. The root-mean-square (RMS) value of surface roughness for the composite film estimated from the atomic force microscope image was 42.7nm. Compared with 85.9nm for the MCD film. And it was also found that the thermal diffusivity increased from 32.61mm2/s to 37.63mm2/s by further growing a NCD film. Results indicated that the deposition of NCD film reduced the rough surface of the MCD film with grain sizes of the order of microns, and thus increased the efficiency of diamond films as thermal spreading device. It was found that the NCD/MCD composite film had a smoother surface and a higher thermal diffusivity compared with MCD film.