Creep tests of T92/Super304H joints were performed at 923 K under the stress of 85–165 MPa. Microstructure evolution was characterized by light microscopy, scanning electron microcopy and transmission electron microscopy to probe the relationship between creep performance deterioration and microstructure evolution. Results showed that for all the creep tests, failure occurred at fine-grained heat-affected zone of T92, and the joints have lower creep strength than the base metal T92. However, as the stress increased from 85 to 165 MPa, the creep fracture changed from a mixed mode, i.e., intergranular fracture in the center part and transgranular fracture in the edge part to total transgranular fracture. The longer the creep life, the greater is the proportion of the intergranular fracture. The M23C6 coarsened and the Laves phase precipitated along grain boundaries during long-term creep. Vacancies nucleate and propagate at the interface between coarse M23C6, Laves phase and matrix. Finally, cracks forming along grain boundaries are responsible for intergranular fracture.