A study of Grade 91 steel's creep rupture behavior at 600°C (up to 90,000 hours) and 650°C (up to 23,000 hours) reveals that static recovery of tempered martensite lath structures leads to decreased stress exponent and breakdown of creep strength. While M 23 C 6 and MX particles initially stabilize lath structures by hindering sub-boundary migration, the progressive aggregation of M 23 C 6 particles reduces their pinning force, triggering static recovery. Although Grade 91 steel shows better M 23 C 6 thermal stability compared to Grade 122 type steels (9-12%Cr-2W-0.4Mo-1Cu-VNb), coarsening of M 23 C 6 particles and subgrain width is expected to occur slightly beyond 100,000 hours at 600°C, potentially leading to creep strength breakdown.