We report that the grain size of an extruded Mg–6Zn–0.5Zr alloy is significantly reduced by the addition of Ca. The grain size was reduced from 4.2 to 1.6 µm by the addition of 0.2 wt% Ca, leading to an increase in the yield strength to as high as 65 MPa. Microstructural analysis indicated that adding Ca to the alloy results in the co-presence of MgZn2 and Ca2Mg6Zn3 particles within the α-Mg matrix and the co-segregation of Zn and Ca at the grain boundaries. The factors responsible for such remarkable changes in the microstructure and tensile properties are also discussed.