Magnesium alloys with homogeneous degradation and controlled degradation rate are desirable for biodegradable materials. In the present work, Mg-3 wt.%Zn-0.2 wt.%Ca alloys with different columnar structures were fabricated and the degradation in 0.9 wt.% NaCl were investigated. With the increase of the growth rate for the directional solidification, the microstructure of the directionally solidified (DSed) alloy evolved from cellular to dendritic coupled with the change of the spacing of the primary trunks (λ₁) and the volume fraction (f_v) of Ca₂Mg₆Zn₃ phase. The results of the corrosion test suggested that the alloy with cellular structure experienced homogeneous corrosion and exhibited the lowest corrosion rate. The good corrosion resistance of the alloy with cellular structure was attributed to the protective corrosion products film (CPF), which was closely related to the f_v of Ca₂Mg₆Zn₃ phase and λ₁. To evaluate the corrosion rates (CR) of the DSed Mg-Zn-Ca alloys with different microstructures, a parameter α was proposed in this work, which was calculated by λ₁ and the f_v of Ca₂Mg₆Zn₃ phase. The fitting result showed that there was a linear relationship between CR and α, which was CR = 4.1899 + 0.00432α. This means that the CR of the DSed Mg-Zn-Ca alloy can be evaluated if the microstructure had been characterized.