The effect of heat treatment on microstructure and property of Ti-45 and 47Al-2Nb-2Mn+0.8%vol.%TiB2 alloys (45XD and 47XD) has been studied. Annealing and subsequent oil quench produced fine-grained fully lamellar structure (FGFL) in both alloys. For microstructural stabilization, the FGFL structures were subjected to different aging treatments. Microstructural examination showed that degradation of the FGFL structure, such as coarsening of γ lamellae, recrystallization of γ grains and break-up of a2 lamellae, presented to varying degrees after different aging treatments. Hardness values in the aged alloys decreased due to the degradation. The creep resistances were improved in the aged alloys though the degradation occurred, indicating that the aging treatments stabilized the FGFL structures effectively. The differences in the changes of properties caused by different aging treatments and compositions were compared in combination with the microstructural variants.