This study deals with influence of temperature and time of austenitization on final mechanical properties, especially Rockwell hardness (HRC), obtained after heat treatment of steel 100CrMn6 (1.3520). Following process parameters of heat treatment were varied: time of austenitization and type of cooling medium, which has a significant effect on final strucutre, thus mechanical properties. These parameters were varied according to reccomended range given in material sheet. As can be seen from results, different temperature and time of austenitization influenced final hardness after hardening and tempering. Lower temperature and shorter time of austenitization led to insufficient homogenization of austenite and incomplete transformation to martensite and bainite, which resulted in lower hardness. On the other hand, higher temperature and longer time of austenitization can lead to coarser austenite grain, which can also result in lower hardness.