Abstract: To explore the potential of high-thermal-stability nanocrystalline Fe-based alloys for intermediate/high-temperature applications, a ferritic-austenitic dual-phase nanocrystalline/ultrafine-grained (NC/UFG) Fe-5 wt.% Mn-8 wt.% Zr alloy was fabricated via mechanical alloying combined with spark plasma sintering. The phase composition, microstructural evolution, and deformation mechanisms of the alloy were investigated through hot compression and annealing experiments within the temperature range of 400-700 °C. The results indicate that the as-sintered alloy exhibits an average grain size of 66 nm, comprising both NC and UFG constituents, with an austenite content of approximately 70 vol.%. Following annealing or deformation at intermediate temperatures, the austenite content decreases and stabilizes above 500 °C, while grain growth proceeds with increasing temperature. The yield strength under hot compression declines from 1100 MPa to 412 MPa as the temperature rises. The deformation mechanism is closely related to grain size: at 400-500 °C, where NC grains dominate, deformation is primarily governed by grain rotation; at 600 °C, where UFG prevail, deformation relies on the synergistic effects of dislocation motion and grain boundary sliding. This study clarifies the influence of grain size on the deformation mechanisms of the alloy at intermediate temperatures, providing theoretical support for its potential application in medium-temperature structural components.