The surface morphology, film quality, interface structure. growth rate and adhesion of diamond films prepared by hot filament chemical vapor deposition on commercial steel substrates with and without ion beam nitriding pretreatment were investigated by scanning electron microscopy, x-ray diffraction. scanning Auger spectroscopy, a Vickers microindenter, and microRaman spectroscopy, Apart from quite different surface morphologies, both the quality and growth rate of diamond films deposited on the ion beam nitrided steel substrates were higher than those on the unnitrided steel substrates. Similar diamond/steel interface structure were observed on both the nitrided and unnitrided steel. Diamond films were grown on steel substrates via carbide interlayers formed in situ during, the diamond deposition rather than via a graphitic soot interlayer. Ion beam nitriding appeared to enhance the diamond growth through thickening of the carbide interlayer. The adhesion of diamond films on the nitrided steel was tested by a Vickers microindenter,