The microstructural evolution and tensile properties of a forged Ti-42Al-5Mn alloy subjected to different heat treatments were studied. The results showed that, when the forged alloy was aged at 800 °C for 24 h, the interlamellar spacing (λ) and γ grain size at colony boundaries are generally coarsened. Whereas, when the alloy was first annealed at 1300 °C and then aged at 800 °C for 24 h, this coarsening of related microstructures appears less pronounced. The suggested annealing temperatures for the forged Ti-42Al-5Mn alloy are in the range of 1250-1300 °C. It was found that, on the condition of the same annealing system, both the strength and ductility were improved as the aging temperature changed from 1000 to 800 °C. The secondary precipitated β_o (β_o,sec) at colony boundaries could be responsible for improving the strength, and the γ phase at colony boundaries with the grain size about 6 μm might be one of the main reasons for the better ductility.