There are few relevant literatures that introduce the lattice constant (α) of W-phase containing Mg, Zn and Er. But the W-phase in Mg95.9Zn3.5Gd0.6 alloy which is FCC structure with composition of 31.21 at% Mg, 43.05 at% Zn and 25.74 at% Gd and its lattice parameter of α = 0.768 nm is confirmed by Liu et al. [3]. It can be seen that the lattice constant (α) of W-phase containing Mg, Zn and Er is lower than that containing Mg, Zn and Gd. The reason for this difference might be that the atomic radius of Gd is larger than that of Er based on the periodic table of the elements.In this study, the concentration of Zn in LZ105 alloy was ~1.6 at%. According to Mg-Li-Zn ternary phase diagram, the concentration of Zn in β-Li phase is more than that in α-Mg phase. And due to the ~10.2 wt % Li addition, the most of matrix is β-Li phase. Therefore, the contribution of solid solution strengthening from Zn concentration isn’t ignored in Mg-Li-Zn alloys with high Li content. It is known that in series of Mg-Li-Zn alloys, the alloys could be strengthened by MgLi2Zn phase, however, this phase is a metastable structure easy to decompose partly into soft MgLiZn phase [25]. So the as-cast LZ105 alloy's main strengthening way is solid solution strengthening of Zn element and its strength is better than that of some other Mg-Li-Zn alloys with lower Li and Zn addition [26,27] due to high solid solubility of Zn in β-Li phase. As for the effect of Er content on the mechanical properties of LZ105 alloy, the results show that a small number of Er addition is effective for the strength improvement of LZ105 alloy while too much Er content is harmful. The mechanism of these can be classified into the followingthree aspects.Firstly, grains of LZ105 alloys are significantly refined with the addition of Er element. Based on Hall-Petch relation, the alloy with higher Er addition should have higher YS than that of alloys with lower or none Er addition. However, the tensile results show that the YS of LZ105-3.5Er alloy with the most Er addition is the lowest among LZ105-xEr alloys. Hence the strength of as-cast LZ105-xEr alloys doesn’t only rely on the grain size. Beside grain refinement strengthening, the second phase strengthening and solid solution strengthening can also play an important role in the mechanical properties of LZ105-xEr alloys.