Small additions of microalloying elements such as niobium, vanadium and titanium have been known to play an important role in the grain refinement, but the role of molybdenum is not clearly known yet. On this study, the effects of molybdenum on the structure of the direct quenched plate steels were investigated. The effect of molybdenum on the prior austenite grain refinement was negligible, but it prevented the recrystallization and made T_R(Recrystallization finishing temperature) lower about 150℃(1150℃→1000℃), and Ar₃ temperature increased. The results were attributed to the pinning effect of the molybdenum precipitates and a broaden ferrite range with molybdenum. A fine-grained steel could be produced through the rolling at the temperature below the T_R, and direct quenching.