Gleeble weld thermal cycle simulation was used to produce individual sub-zones (SZs) of the Heat Affected Zone (HAZ) in P91 steel for microstructural analysis, hardness testing and accelerated creep testing. Simulated post-weld heat treatment (PWHT) was applied to the simulation samples before creep testing. Notched creep test-pieces were used to simulate the triaxial stress state expected under service conditions. The creep results emphatically confirmed the well-known susceptibility of the Intercritical (IC) and Grain Refined (GR) Heat Affected Zone structures to premature Type IV creep fracture. In contrast, the simulated Grain Coarsened (GC) structure showed even higher creep resistance than the normalised and tempered (NT) base steel and creep testpieces of this structure did not fail under test before Type IV fracture occurred in the shoulder region of the test-piece where the peak temperatures during simulation corresponded to those of the ICHAZ and GRHAZ. It is concluded that the weld thermal cycle and subsequent PWHT of the ICHAZ and GRHAZ compromise precipitate and dislocation strengthening and boundary pinning, severely degrading the creep resistance.