The microstructure and mechanical properties of austempered ductile cast iron (ADI) are substantially influenced, above all, by the conditions of isothermal transformation which have extraordinary crucial influence on the resulting composition of the structural mixture of matrix and, as a consequence, on all its mechanical properties. In the present paper the influence of isothermal transformation temperature changing in the range from 380 to 500 °C on the structure, tensile and fatigue properties of unalloyed nodular cast iron is studied. Mechanical tests were supplied by metallographic analysis and quantitative X-ray phase analysis.