Abstract: Diamond thin films have been deposited by the hot filament CVD method on molybdenum substrates from the mixture reactant gas of acetone and hydrogen. The surface morphology of the obtained diamond films under various deposition conditions, i. e. , different reaction pressures, was observed by scanning electron microscope (SEM). The experiment results strongly indicate that the surface morphologies of the resulting films have closely related to the reaction pressures. Under the lower reaction pressure, the grains consisting of the resulting films present mainly the morphology similar to that of the small single crystal cubo-octahedron and double small crystal grains. Under the higher reaction pressure, however, the surface morphology of the grains looks like the large cauliflower. These results indicated that there exist, different growth mechanisms for CVD diamond crystals under various deposition conditions finally, the structure of cauliflower-like diamond grains was been analyzed and the formation mechanism studied.