The effects of austenitizing conditions, such as austenitizing temperature, austenitizing time and the austenitizing temperature in previous heat treatment, on the hardenability and the mechanical properties of hardened eutectoid carbon steel were determined, and the relations between those properties and the austenitic grain size were ascertained, with the following results: (i) The higher the austenitizing temperature, the lower the toughness of hardened steel, the greater its hardenability, and the coarser its austenitic grain size; (ii) a longer austenitizing time gives the same effect as higher austenitizing temperature on those properties, but in some cases, an abrupt change in toughness and hardenability unrelated with austenitic grain size was observed; (iii) even if the austenitizing temperature in final hardening was maintained constant (800°C), the toughness and the hardenability were remarkably affected by the austenitizing temperature in previous heat treatment, such as annealing, hardening and annealing after hardening, where the higher was the austenitizing temperature, the toughness fell the lower and the hardenability rose the higher. But, in all cases, the austenitic grain size was scarcely affected by previous heat treatment; (iv) the above results suggest that decrease in toughness and increase in hardenability due to overheating would be attributed to some change in austenite produced by the overheating process itself, in addition to austenitic grain coarsening.