The high ductility of Mg-Li alloy has been mainly ascribed to a high activity of pyramidal <c+a> slip to accommodate plastic strain. In the present study, however, a quantitative analysis reveals that Li-addition can only slightly stimulate the activation of pyramidal <c+a> slip under compression along the normal direction of a hot-rolled Mg-4.5 wt.% Li plate, with a relative activity of approximately 18%. Although the limited activity of pyramidal <c+a> slip alone cannot accommodate a large plastic strain, it effectively reduces the number of {1011} − {1012} double twins, which are believed to be favorable sites for crack initiation. The evidently reduced activity of double twins leads to a lower cracking tendency, and therefore improves ductility.