It is significant to investigate the effect of La2O3 as milling aid and hydrogen sorption catalyst for magnesium in order to solve its problem of low kinetic performance. The XRD analysis shows that after milling with 1.0 mol.% of La2O3, the crystallite size of Mg decreases to 49.7 nm, while it is 51.6 nm without La2O3. The decrease in crystallite size from the addition of La2O3 is in favor of the improvement of hydrogen sorption rate of Mg. The SEM observation shows that La2O3 nanocryatallites contact with crystalline Mg closely and form mosaic structure, which is in favor of La2O3 playing the role of catalysis for hydrogen sorption of Mg. The kinetics test shows that with the addition of La2O3, the hydrogen sorption rate of Mg increases obviuosly. With the addition of La2O3, the activated energy of hydrogen absorption decreases by 49.8 kJ/mol, and the activated energy of hydrogen desorption decreases by 23.1 kJ/mol. So, the kinetic performance of hydrogen sorption of Mg is substantially improved with the addition of La2O3. The p-c-T isothermal absorption analysis shows that the hydrogen absorption activity of Mg increases and results in lower equilibrium hydrogen pressure. The DSC analysis shows that the hydrogen desorption rate of MgH2 decreases with the addition of La2O3. In the process of milling, nanoscale La2O3 play the role of milling aid with its higher hardness than that of Mg, and it does not react with Mg; in the process of hydrogenation and dehydrogenation, La2O3 plays the role of catalyst, and it does not react with H2.