This paper is focused on the study of the effect of high heating rates on austenite formation of 18NiCrMo5 steel. The prior martensitic microstructure was characterized with scanning electron microscopy and Vicker micro-hardness. Then dilatometric tests were performed at 12 different heating rates (in the range of 100 - 3000°C/s), to evaluate the critical points and the key factor governing the austenite formation with high heating rates. After these tests, the microstructures obtained were analyzed...