The investigation results of the thermal fatigue resistance of two ADI grades (EN-GJS-1200-2 and EN-GJS-800-8) are presented in the paper. Tests were performed on the author’s research stand, by means of the resistance heating of samples acc. L.F. Coffin method. The thermal fatigue resistance is the basic criterion in assessing the material suitability for metal moulds. The maximal temperature (Tmax) of thermal cycles influence on their number, which the sample can withstand before cracking, were estimated. Structure transformations, hardness and strength changes of the austempered ductile iron (ADI) were analysed. It was found, that at cyclic heating to Tmax <500°C, the ADI retains its primary ausferritic structure, even after more than 10.000 thermal cycles. Such ADI behaviour predisposes it to be applied as a material for metal moulds used in the pressure die casting, it means mainly at casting Mg and Zn alloys as well as for small castings of Al alloys.