Post-irradiation tensile properties of Ti-rich TiAl (approximately Ti-47at% Al) intermetallic alloys, produced from the powder product manufactured by a plasma rotating electrode process (PREP), are investigated. The TiAl alloy powders are hot isostatic pressed (HIPed) into a cylindrical large compact, and then consolidated and formed by isothermal hot-forging (IHF). The final duplex structure of the alloys consists of equiaxial grains of the Ti 3 Al and TiAl(γ) phases with a grain size of approximately 2 μm in diameter.An increase in ductility from 6.0 to 10.3% with a slight increase in yield strength, is observed in specimens tested at 873 K after neutron irradiation at 873 K to a dose of 1 x 10 2 0 n cm - 2 (E > 1 MeV) in JRR-2. This phenomenon can be understood by a mechanism that involves twins, which are believed to play an important role in the deformation process, nucleating during irradiation and growing during tensile testing after irradiation.