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We employ a ternary eutectic Mg 76.87 Ni 12.78 Y 10.35 alloy to achieve a fine eutectic structure with long-period stacking ordered (LPSO) structure, and investigate the corresponding hydrogen storage behavior. The as-cast Mg 76.87 Ni 12.78 Y 10.35 alloy is composed of Mg 15 NiY, Mg 2 Ni, MgNi 4 Y and Mg phases. A high density of stacking faults and 14H-type LPSO structures form in the Mg 15 NiY phase. This fine ternary eutectic structure has shown significant improvement in promotion of the activation and following hydrogen absorption kinetics compared with as-cast non-eutectic structure. Pressure-Composition-Temperature (PCT) curves demonstrate two major pressure platforms due to the presence of Mg 15 NiY and Mg 2 Ni in our Mg–Ni–Y alloys. Nanosized Mg 2 NiH x , MgH 2 and YH x are in situ generated from the decomposition of LPSO structure during hydrogenation. The fine ternary eutectic structure and dispersed nanocatalysts from decomposition of LPSO structure synergistically facilitate the activation and hydrogen absorption kinetics of ternary eutectic Mg–Ni–Y alloy. Image 1 • A fine ternary eutectic Mg–Ni–Y hydrogen storage alloy is designed and prepared. • A 14H-type long-period stacking ordered (LPSO) structure is induced in this alloy. • In situ dispersed nanocatalysts are produced in LPSO during hydrogenation. • Activation and kinetics are improved by ternary eutectic microstructure with LPSO. • Hydrogenation mechanism of ternary eutectic Mg–Ni–Y alloy with LPSO is proposed. [ABSTRACT FROM AUTHOR]