The effect of grain boundary segregation on plastic deformation was investigated using the Mg-Y solid solution binary alloy. Deformed microstructural observations revealed many traces of prismatic < a > dislocations as well as basal dislocations. These dislocations were nucleated at grain boundaries with segregation of yttrium element. By comparison of material factors of binary alloy, the alloying elements having low critical resolved shear stress (CRSS) of non-basal plane and large grain boundary (twin boundary) segregation energy led to activation of non-basal dislocations in the vicinity of grain boundaries. The Mg-Ca alloy had similar material factors and showed the same deformed microstructures as those of the alloys containing rare-earth element, which indicate that calcium element is an alternative alloying element.