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 $900^{circ}C$ to $950^{circ}C$, whereas they grow rapidly in the temperature range of $975^{circ}C$ to $1100^{circ}C$. (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 $900^{circ}C$. (3) The hardenability improves ramarkably as Mn content is increased at three different austenitizing temperatures $900^{circ}C$, $1000^{circ}C$ and $1100^{circ}C$. (4) The maximum hardenability is obtained from steels A, B and C austenitized at the $900^{circ}C$, although Mn content is varied in each specimen.