[en] The evolution of phase constituent of Mg_99.0-xNi_xY_1.0 (x = 0.2, 0.5, 0.8, 1.0, 1.5, at.%) alloys was investigated by OM, SEM/EDS, TEM and XRD. Apart from the α-Mg phase, the secondary phases in these ternary alloys mainly consist of LPSO (long period stacking ordered) structure, Mg₂₄Y₅ phase or Mg₂Ni phase, which depend on the atomic ratio of Ni and Y. The amount of LPSO structure in the alloys increases firstly and then decreases with the increase of Ni level, but the change in the amount of LPSO structure is not proportional to that of Ni level. In the as-cast alloys containing excessive Ni, there exist two forms of LPSO structures with different characteristics of bulk and lamellar shapes, respectively. During solidification process, part of the bulk 18R-type LPSO structure formed firstly is transformed to the lamellar 14H-type LPSO structure. The tensile properties of different alloys are tested and the changing trend of strength is similar to that of the amount of LPSO structure. When the Ni content is 0.5 at.%, the ultimate tensile strength, yield stress and elongation are 208 MPa, 93 MPa and 8.0%, respectively, which are the optimal properties among these alloys. - Highlights: • Effect of Ni content on the phase constituents of Mg_99-xNi_xY₁ alloys was studied. • Part of 18R-type LPSO is transformed to 14H during solidification. • The optimal mechanical properties is obtained when Ni content is 0.5 at.%.