Mg77+xNi20−xLa3(x= 0, 5, 10, 15) alloys were successfully prepared by the vacuum induction melting method. The structural characterizations of the alloys were performed by using X-ray diffraction and scanning electron microscope. The effects of nickel content on the microstructure and hydrogen storage kinetic of the as-cast alloys were investigated. The results showed that the alloys are composed of a primary phase of Mg2Ni, lamella eutectic composites of Mg + Mg2Ni, and some amount of LaMg12and La2Mg17. Nickel addition significantly improved the hydrogen-absorption kinetic performance of the alloy. At 683 K, Mg77Ni20La3alloy and Mg82Ni15La3alloy underwent hydrogen absorption and desorption reactions for 2 h, respectively, and their hydrogen absorption and desorption capacities were 4.16 wt.% and 4.1 wt.%, and 4.92 wt.% and 4.69 wt.%, respectively. Using the Kissinger equation, it was calculated that the activation energy values of Mg77Ni20La3, Mg82Ni15La3, Mg87Ni10La3and Mg92Ni5La3alloys were in the range of 68.5~75.2 kJ/mol, much lower than 150~160 kJ/mol of MgH2.