Abstract: The effect of tensile strain rate on deformation microstructure was investigated in Ti-6-4 (Ti–6Al–4V) and SP700 (Ti–4.5Al–3V–2Mo–2Fe) of the duplex titanium alloys. Below a strain rate of 10−2 s−1, Ti-6-4 alloy had a higher ultimate tensile strength than SP700 alloy. However, the yield strength of SP700 was consistently greater than Ti-6-4 at different strain rates. The ductility of SP700 alloy associated with twin formation (especially at the slow strain rate of 10−4 s−1), always exceeded that of Ti-6-4 alloy at different strain rates. It is caused by a large quantity of deformation twins took place in the α phase of SP700 due to the lower stacking fault energy by the β stabilizer of molybdenum alloying. In addition, the local deformation more was imposed on the α grains from the surrounding β-rich grains by redistributing strain as the strain rate decreased in SP700 duplex alloy.