Abstract: In the paper the fracture of spindles of milling machine is referred as an example to the clarification of fracture mechanical analysis applying in the brittle fracture cases of components made from nodular cast iron. In terms of the conventional strength estimation, if there would not be any cracks, the strength of the spindle should be strong enough, and no failures would ever take place. Since the crack-like defects have been found at the critical zone on the fracture section surface of the damaged spindle, thereby the supplementary fracture mechanical analysis should be taken into consideration. The analysis indicates that the spindle works reliably when running stably under rated torque even at the presence of some casting defects owing to the very low applied stress. In consequence of the adverse residual stress produced by heat treatment and unexpected overloading, the stress in critical zone increases steeply, the unstable crack propagation takes place and brings forth brittle fracture. Then the improvement of heat treatment has been studied, the results indicate that by decreasing the austenitizing temperature of the initial quenching and tempering treatment as well as adopting the isothermal transformation process, the fracture toughness value may be elevated from 76～94 kg·mm-3/2 to 111～127 kg·mm-3/2 with a favourable state of residual stress, and the failures would have been essentially prevented.