Abstract: Fine-grained Super304H tube has been widely used in superheaters and reheaters of ultra（supercritical） units.However, the formation and control mechanism of the fine-grained microstructure are still unclear. The effects of hightemperature softening and rolling deformation on the grain growth behavior and distribution of nanoscale MX phase of Super304H heat-resistant steel were systematically studied. The results show that the MX phase partially dissolves into the austenite matrix after high-temperature softening. During the final solution treatment, the MX phase precipitates in nanoscale form. The bigger the rolling deformation is, the more dispersive the nanoscale MX phase is, the stronger the pinning effect of MX on grain boundary migration is, the more obvious the depressing effect on grain growth is. When the rolling deformation is 56%, the grain size is still controlled below 20 μm after final solution treatment for 100 min. The Super304H without high-temperature softening does not have nanoscale MX phase precipitation during the final solution treatment, and the gran size is 25-40 μm after 0 min to 100 min of final solution treatment process.