Hydrogen storage properties and phase components of Mg-doped TiFe alloys, that were prepared by Ti, Fe and Mg metal powders using a mechanical alloying technique, were studied. XRD analyses show that the main phase of all the Mg-doped Ti1.2Fe alloys is the TiFe phase. Some TiFe2 phase and α-Ti phase exist as secondary phases and Mg is dispersed in the alloy matrix. 3% Mg-doped and 5% Mg-doped Ti1.2Fe alloy samples can be fully activated within three hydriding/dehydriding cycles at room temperature and the hydrogen storage capacities of the alloys can reach 222mL/g and 198mL/g, respectively. Both two samples exhibit only one plateau region in their P-C-T curves with a low hydrogen absorption/desorption pressure hysteresis. The effect and mechanism of Mg addition as well as overstoichiometric Ti on the activation properties and hydrogen storage capacities of the alloys was also discussed.