The aim of the present work is to investigate the microstructural behavior of austenite grain size (AGS) during the reheating process of two different API steel grades (X65 and X70). The steel samples were austenitized at 1150ºC, 1200ºC and 1250ºC for various holding times from 10 to 60 minutes and quenched in ice water. The samples were then annealed at 500ºC for 24 hours to reveal the prior AGS using optical microscopy. It was noticed that the AGS in X65 grade is coarser than that of X70 grade.