In recent years the Hydrogen was considered as fuel by many researchers and scientists. Given that solid mode storage of Hydrogen is the safest and the most productive mode, many projects have been defined in the field of improving Hydrogen properties of the metals with the most capacity of Hydrogen Storage. The compositions of materials contain Niobium were used in many researches and the desired results were presented in multiple articles. According to the similarities of Tantalum to Niobium in chemical and physical properties, and by considering that there are rare researches about the effect of compositions of Ta on Hydrogen properties of metals, the decision of studying effects of Tantalum Oxide on MgH2 as base metal was made after reading many articles about the compositions of Tantalum. In this research, the composite materials, with the compositions of MgH2-10%wt Ta2O5, MT1 samples; and MgH2-20%wt Ta2O5, MT2 samples; were prepared by ball milling of Magnesium Hydride and tantalum oxide powders. Effect of additive and ball milling time on structure of magnesium Hydride were evaluated, contains grain size, net strain and particle size and also the dehydrogenation properties of prepared composites. And were compared with ball milled pure Magnesium Hydride. It has been shown that the addition of Tantalum Oxide to Magnesium Hydride at the primary time of ball milling cause reduction in particle size from 10.6 to 0.27 μm in  ball milled MgH2-10%wt Ta2O5 for 3 hours (MT1-3 sample) and to 0.25 μm in ball milled MgH2-20%wt Ta2O5 for 3 hours (MT2-3 sample). As a result, the dehydrogenation temperature of mechanical activated magnesium Hydride after 3 hours were reduced from 405 Cº to 382 Cº for MT1-3 and to 370 Cº for MT2-3. The change in amount of temperature reduction depends on numerous reasons such as the percent of catalyst and the metallic phase that has been formed during ball milling.