In this work, the research results of heating up rate influence on S355J2G3 steel austenitization temperatures are presented. Dilatometric tests were carried out on the thermal cycle simulator Smitweld TCS1405 for a heating speed ranging from 1 to 150 0 C s -1 . On the basis of the achieved results, the functions associating start and end temperatures of austenitization with the heating up speed are proposed.