Effects of Si on the softening of the squeeze cast Mg-10wt%Li-3wt%Al allay have been investigated as functions of Si content, solution treatment, aging temperature and time. Microstructures of as-cast Mg-10wt%Li-3wt%Al-Xwt%Si(X:0.2, 0.4, 0.6, 1.0, 1.2) alloys consist of a mixture of α, β, AlLi and Mg₂Si phases. The hardness increased with the addition of Si due to the formation of polygon and Chinese script type Mg₂Si in the α and β phase and due to the decrease of the amount of AlLi phase. In the case of alloys having Si content lower then 0.4wt%, two types of Mg₂Si became much finer, and the volumes of ㎦ and AlLi phases decreased with the increase of the aging temperature and time which resulted in the improvement of hardness. It Can be concluded that the combined effects of fine dispersion of Mg₂Si, disolving of Al into the matrix, and decreased volume of α and AlLi effectively lead to the prevention of softening.