The thermo-mechanically processed powder Ti-5Al-5V-5Mo-1Cr-1Fe alloy was aged at 9.23 K for 1-8 h in order to investigate the effect of ageing time on the microstructure-mechanical properties relationships. The microstructures of the alloy after ageing cosist of the both primary and secondary alpha phases with retained B matrix phase. The shift of B peaks in X-ray pattern with ageing time indicates an increase in lattice parameter distorition due to diffusion into the retained B phase of more B stabilisers with smaller atomic radius compared to the elemental Ti. The tensile test results indicate that the sample aged for 1 h has achieved the best combination of mechanical properties with ultimate tensile strength of 1194 MPa and total elongation of 14/1% among all the experimented conditioins. The modified Crussard-Jaoul method is applied to characterise the work hardening behaviour of the alloy.