Abstract: To investigate the spallation of oxide scales on ferritic heat-resistant steel T91 under cyclic loading conditions, this study examines the steam oxidation behavior of both long-term serviced and as-received T91 steel samples from boiler applications. The mass changes of the heat-resistant steel before and after oxidation are measured using an analytical balance. The morphology and composition of the oxide products are characterized using scanning electron microscopy and X-ray diffraction. Comparative experimental results demonstrate that the weight gain of the supplied state T91 is greater than that of the long-term serviced T91 under both stable and variable load conditions at 600℃ and 625℃. Compared to stable loading, the variation in atmosphere is caused by load variation resulted in a sudden change in stress on the oxide film of the ferritic heat-resistant steel T91, leading to severe cracking of the oxide film. The long-term serviced samples experience more severe scale peeling due to the presence of numerous voids and structural defects. Boiler operation under load variation can result in the peeling of the oxide film and increase the risk of tube blockage in pressure-bearing components.