This study has been carried out to investigate into some effects of Mn content with varying amounts and austenite grain size on hardenability in boron-added Fe-C-Cr-Mo alloy systems. (1) Austenite grains have been found to hardly grow in the temperature range of 900℃ to 950℃, whereas they grow rapidly in the temperature range of 975℃ to 1100℃. (2) Austenite grain growth is considerably small with increasing holding time at a given austenitizing temperature and is, in particular, hardly found to occur at a temperature of 900℃. (3) The hardenability improves ramarkably as Mn content is increased at three different austenitizing temperatures 900℃, 1000℃ and 1100℃. (4) The maximum hardenability is obtained from steels A, B and C austenitized at the 900℃, although Mn content is varied in each specimen.