<p>This study investigates the effect of cobalt content on the microstructure and creep rupture life of 10 wt% Cr ferritic/martensitic heat-resistant steels. Two alloys were fabricated with different cobalt contents (3.7 wt% and 2.7 wt%), while maintaining similar concentrations of other alloying elements. Microstructural characterization revealed that the Low-Co alloy exhibited finer prior austenite grains and a higher density of M₂₃C₆ and Laves precipitates compared to the High-Co alloy. Creep tests conducted at 650&#xa0;°C under 145&#xa0;MPa demonstrated that the Low-Co alloy achieved a superior creep life (4518&#xa0;h) than the High-Co alloy (3846&#xa0;h). The improved creep performance was attributed to enhanced microstructural stability in the Low-Co alloy, including greater pinning of dislocations and suppression of subgrain coarsening. These results suggest that optimizing cobalt content is crucial for balancing beneficial effects on microstructure and long-term creep resistance in advanced heat-resistant steels.</p> Graphical Abstract <p></p>