The tensile deformation mechanism of α 2 -phase (DO 1 9 ) in a Ti-25Al-10Nb-3V-1Mo alloy at 25 -- 700 o C was studied. In accordance with g . b = 0 invisibility criterion, dislocation types and slip systems were analysed in a TEM equipped with a side entry goniometer stage capable of a +/- 45 o tilt on X- and Y-axis operating under two-beam condition. Experimental results show that the majority of slip systems activated at 25 -- 700 o C are a type dislocation on prismatic and basal planes. A few ofc + a type dislocation pairs on pyramidal plane are found at 25 o C, so a large amount of twinning found at 400 o C. The density ofc + a type dislocation pairs decrease with the increase of temperature. Noc + a type dislocation pair is found at 700 o C, but some a type dislocation pairs are activated at 700 o C.