The O-phase (orthorhombic Ti2AlNb phase) intermetallic alloy has been considered one of the strongest materials for high temperature application. The primary compositions of the test alloys in this paper are Ti-22Al-27Nb (at.%) and Ti-22Al-20Nb-2W (at.%). By in-situ X-ray diffraction (XRD) tests, each phase is identified and the lattice parameters of each phase are calculated for different temperatures. XRD tests show that a T2AlNb-beta phase, partially ordered b.c.c. phase, exists besides O, alpha(2) and B2 that are usually expected in the Ti-Al-Nb alloy system. In this paper, the effects of tungsten on the microstructure and micro-hardness of a Tl2AlNb-based (O+b.c.c.) alloy are investigated. The effects of W on Ti2AlNb-based intermetallic alloys are widely known to cause a refinement of microstructure and improvement of mechanical strength, however, the reason has not yet been reported. In this paper, the role of W in the improvement of mechanical strength is clarified. The main effect of W addition is not the formation of a solid solution or the formation of W-precipitates but the refinement of the Widmanstatten structure. (C) 2002 Elsevier Science B.V. All rights reserved.