In this work, three amphoteric metal oxides (Al2O3, TiO2 and Fe2O3) are introduced into Mg ultrafine powders via arc plasma method in order to improve hydrogen absorption properties of Mg and hydrolysis performances of MgH2. The phase components, particle size distribution, microstructures and compositions of pure Mg and coreshell structured Mg-MOx nanocomposites at various states are carefully characterized. Among all composites, the addition of TiO2 followed by passivation shows the best promotion effects on hydrogen absorption and hydrolysis performances. The passivated Mg-TiO2 composite absorbs 5.5 wt% of H-2 in less than 5 min at 623 K and the hydrogenated composite produces more than 1000 mL g(-1) H-2 in 660 s (maximum 1525 mL g(-1) for 1 h) in 0.1M MgCl2 water solution at 298 K. The activation energy of hydrolysis decreases from 53.42 kJ mol(-1) H-2 for hydrogenated pure Mg powder (passivated) to 45.14 kJ mol(-1) H-2 for the hydrogenated Mg-TiO2 powder (passivated). Corresponding analyses reveal that TiO2 formed on the surface of Mg particles, together with the defects in Mg lattice accounts for the improved hydrogen absorption properties, the enhanced hydrolysis rate and yield of the hydrogenated Mg-TiO2 powder.