Abstract: Microstructures of the heat-affected zone (HAZ) of a Gr. 91 steel weld were simulated to evaluate their effects on the creep life of the weld at elevated temperatures. The Ac1 and Ac3 temperatures of the Gr. 91 steel were determined by a dilatometer to be at 867 and 907 °C, respectively. An infrared heating system was employed to heat the samples to 860 (STOT), 900 (ICHAZ) and 940 °C (FGHAZ) for 1 min, followed by cooling to room temperature. The simulated specimens were then subjected to conventional post-weld heat treatment (PWHT) at 750 °C/2 h. After the PWHT, the tempered ICHAZ specimen had a shortest creep life among the specimens tested at 650 °C/60 MPa. Moreover, the simulated specimen heated to 860 °C (STOT) was more likely to fracture at 615 °C/80 MPa than others.