Titanium alloy Ti5Al2.5Sn ELI is an important structural material in the field of aerospace and others. Microstructure and mechanical properties of the alloy are studied in the paper by methods of TEM, XRD, and etc. The results indicate that microstructure of Ti5Al2.5Sn ELI alloy is consist of single α phase which is distributed in platelet. After HIP treated, there is no obvious change in the microstructure but the recrystallization on the position of defects closed. The alloy has moderate strength, good plasticity and toughness. The fatigue crack growth rate of the alloy is low, and the accurate value of K_Ic cannot get by testing fracture toughness with specimens of 25 mm in thickness, just the condition value K_q can be gotten. The ability of anti-fracture and crack propagation of ELI grade titanium alloy is higher than non-ELI grade alloy.