The effects of cooling condition on the Zr-rich core formation and grain size in Mg-Zr alloys were investigated in this study. Four moulds with various cooling rates and different Zr additions were used. The results show that when Zr addition was 1.2%, the Zr-rich cores were round and their diameters increased with decreased cooling rates. When Zr addition content was decreased to 0.7%, Zr-rich cores formed only in the sand mould with the lowest cooling rate used in this study, and the morphology changed to rosette-like. The influence of cooling rate on grain size of Mg-Zr alloys was also investigated.