This work focusses on the link between microstructure and creep properties for heat-resistant austenitic alloys with high aluminum content (3–5 wt %). An emphasis was put on the coupling of thermodynamic simulations, microstructural characterizations by scanning electron microscopy and transmission electron microscopy, and creep testing. The phase predictions performed by the calculation of phase diagrams method are in good agreement with the observed microstructure after creep at 1000 °C and 1050 °C. Correlation between creep properties and microstructure characterizations at 1000 °C and 1050 °C revealed that NiAl and α’ (chromium-rich base centered cubic phase) phases are deleterious for the creep properties at service temperature. Several high Al-content alloys are selected in order to replace the chromia-forming alloys standardly used in cracking furnaces.