We report on the microstructure and mechanical properties of T91 alloy subjected to different thermomechanical treatments (TMTs). Equal channel angular extrusion (ECAE) was performed at 23, 300, 625, and 700 A degrees C. Mechanical strength of T91 alloy is enhanced after cold-ECAE at the expense of ductility. In comparison, the hot-ECAE process leads to less pronounced hardening but retention of ductility and work hardening ability. Microstructure analyses reveal that cold-ECAE has significantly reduced the average grain size, while hot-ECAE is more effective to refine and uniformly redistribute carbide nanoprecipitates. Post-ECAE annealing (500 A degrees C/10 h) leads to reprecipitation of large carbide particles in cold-ECAE processed alloy, whereas the hot-ECAEed specimens retain smaller nanoscale carbide precipitates and uniform microstructure. A mechanism is proposed to explain the influence of TMT on the evolution of precipitates. Strengthening of T91 alloy is discussed through a modified model that considers the effects of dislocation density, grain size, and precipitation hardening.