Multielement and multiphase intermetallic alloys based on an ordered orthorhombic (O) phase Ti₂AlNb, where the presence of a long-range order superlattice structure effectively impedes the movement of dislocations and high-temperature diffusion, are a class of highly promising lightweight high-temperature structural materials for aerospace applications due to their high specific strength and superior fracture toughness. Thermal stability of microstructures in the hot rolled sheet of a low-density Ti₂AlNb-based alloy has been investigated in a temperature range from 600^oC to 1100^oC for 12 h via OM, SEM, XRD, and TEM/STEM. The results showed that the initial Ti₂AlNb-based alloy hot rolled sheet consisted of α₂, B2, and O phases. Furthermore, the Ti₂AlNb-based alloy hot rolled sheet at 600^oC for 12 h consisted of α₂, B2, and O phases, where the particle shaped α₂ phase was distributed in the B2 matrix, and lath-like O phase lay inbetween the α₂ particles. The spheroidization of the α₂ phase started to occur along with the coarsening and solutionizing of the lath O phase in the B2 matrix at a temperature between 800^oC and 900^oC for 12 h, while the hot rolled Ti₂AlNb-based alloy plate was still composed of α₂, B2, and O phases. When the temperature reached 950^oC, the O phase disappeared in the B2 matrix. Only α₂ + B2 two phases were present in the hot rolled Ti₂AlNb-based alloy at 950-1000^oC for 12 h, where the α₂ phase was spheroidized and tended to distribute surrounding B2 grain boundaries. When the temperature rose to 1100^oC, the alloy contained a B2 single phase with only some residual α₂ phase. Moreover, the Vickers microhardness contour vs temperature plot revealed that a peak hardness of as high as 509 HV appeared at 600^oC due to the presence of numerous fine O laths.