The microtopography of NiP films ( P content, 8.7–12wt.%; thickness, 40–50 μm ) prepared by electroless plating on 18–8 stainless steel plates for molds and etched with a 10 keV Ar+ ion beam was studied both by SEM observations and Talystep surface profile measurements. With films containing less than 9.6wt.%P, which is supposed to have a transient structure between the supersaturated solid solution and the amorphous solid, smooth surfaces were obtained down to a few micrometers of etch depth and after that craters were formed. As for the mechanism of crater formation, it is considered that Ni crystallites locally precipitate by ion bombardment and then are etched selectively. With amorphous films containing 10.6wt.%P, microcavities less than 0.02 μmRmax instead of craters were observed regardless of the etch depth. The ion beam etching of NiP films has been found to be very promising for the preparation of precision molds of various grades, e.g. for plastic lenses used in opto-electronics, by selecting P content according to the desired surface roughness and etch depth of a particular application.