Using a Gleeble 1500 hot simulator, the dynamic recrystallization (DRX) behavior and changes of DRXgrain size of a Mn-Cr type gear steel have been investigated at deformation temperatures of 900-1150℃, strain rates of 0.1-1 s−1, and initial austenite grain sizes of 70-150m. The experimental results show that the Mn-Cr gear steel exhibits a typical DRX behavior under higher deformation temperatures, lower strain rates and smaller initial austenite grain sizes. The DRX takes place under the conditions with Zener-Hollomon parameter (Z) less than a critical value Zc. The DXR grains are formed by the bulging mechanism. By regression analysis, activation energy and stress exponent were determined to be 378.6kJ/mol and 5.8, respectively. The complete DRX grain size is a power law function of Z with an exponent of-0.25. The reason why Z parameter determines uniquely the DRX grain size was analyzed.