High-strength materials often rely on their fine crystal structure, but the tendency of these crystals to coarsen at high temperatures poses a significant challenge in cutting-edge fields. Enhancing thermal stability while maintaining mechanical properties has become a complex issue in material development. In alumina ceramic fibers, we had observed that the ordered superstructures induced by segregation at the grain boundary networks of α-Al2O3 significantly enhanced the thermal stability of alumina grains without compromising their tensile strength. This led to a remarkable increase in heat-resistant temperature for fine alumina grains, which could exceed 1250 °C and thus greatly expanded the upper limit of alumina materials in high-temperature fields. Simultaneously, the phase-field simulation revealed that ordered superstructures likely reduced alumina's grain boundary diffusion coefficient to nearly match its volume diffusion coefficient. It was foreseeable that these structures would become a promising strategy for enhancing the high-temperature stability of alumina ceramic materials in the future.