The La0.67Mg0.33Ni3 alloys were prepared by magnetic field assisted sintering synthesis (MASS). The hydriding/dehydriding properties and phase structure of the as-prepared materials were investigated through X-ray diffraction (XRD), identical volume method (PCT) and Differential Scanning Calorimetry (DSC). The results show that all the La-Mg-Ni alloys have the major phase of PuNi3 type (La, Mg)Ni3. After hydriding/dehydriding cycles, the phase structure is changed to La2Ni7, MgNi2 and LaNi3 with the lattice expansion. The measurement of PCT curves at room temperature indicates that the sample prepared under 1 T magnetic field has the minimum hydriding/dehydriding hysteresis (0.480), and the maximal dehydriding capacity (1.307wt%). DSC measurement indicates that the sample has two overlapping endothermic peaks which respectively corresponded to the hydrogen desorption of (La, Mg)Ni3 and LaNi5 hydride.