The effects of Zr addition on the mechanical properties and in vitro degradation behavior of Mg-1.0Yb-xZr (x=0, 0.2, 1.0, and 1.53, wt.%) cast alloys were investigated. The results indicated that with increasing Zr addition, a much refined and homogeneous equiaxed grain structure was achieved from a typical columnar grain structure, in companion with the appearance and coarsening of Zr-rich particles. Subsequent electrochemical and immersion tests demonstrated that the corrosion of the alloy was Zr-addition dependent. A trace or excessive Zr addition caused severe localized corrosion attacks, whereas Zr-free and 1.0 Zr alloyed counterparts were generally corroded uniformly. The good combination of mechanical properties and corrosion resistance of the Mg-1.0Yb-1.0Zr alloy was resulted from the refined and homogeneous equiaxed grain structure and fine dispersed Zr-rich particles, thus improving the comprehensive mechanical properties by grain refinement and reducing corrosion rate by generating a more stable and compact passivation layer during long-term immersion.