Effect of hardenability, grain size and microstructural change according to the change of austenitizing temperature was analyzed in Jominy hardenability test of AISI 51B20 steel. Grain growth was small, 7 ${mu}m$ and 12 ${mu}m$ austenite grain sizes at austenitizing temperature of $900^{circ}C$ and $1000^{circ}C$, respectively, while rapid grain growth was observed up to 30 ${mu}m$ austenite grain size at austenitizing temperature of $1100^{circ}C$. As austenitizing temperature increased from $900^{circ}C$ to $1100^{circ}C$, hardenability in the region within 15 mm from end-quenched surface decreased due to the grains growth of bainite and martensite mixture, on the other hand the hardenability in the region exceeding 15 mm from end-quenched surface increased. Increased hardenability was attributed to different microstructures; pearlite, fine pearlite and bainite, and bainite and martensite structures at austenitizing temperature of $900^{circ}C$, $1000^{circ}C$ and $1100^{circ}C$, respectively.