The microstructure and orientation relationship (OR) of the alpha(2) and omega(o) phases in the Ti-34Al-13Nb (at. %) alloy were investigated. The alpha(2) segments nucleated at the omega(o) boundaries after aging at 900 A degrees C for 2 h. The ORs between the two phases were studied by selected area diffraction patterns using a transmission electron microscope and interpreted by the superimposed stereographic projections. Two ORs were defined as follows: ; and ; . Two alpha(2) laths with parallel directions were observed in this alloy, which were explained by the different alpha(2) variants nucleated at the omega(o) boundaries according to ORI and ORII.