Mg-10wt%Gd-3wt%Y alloy was cast in a step-like mould, which provided five different cooling rates. The dentrite morphology and hardness of the as-cast alloy from the surface to the center was investigated and the influence of the cooling rates on these was analyzed. It was indicated that there were two different trends for the hardness distribution: in the section of the step castings with the two slowest cooling rates, the hardness decreased with the increase of depth; while in the other three step castings the hardness increased with the increase of depth. Also it was founded that the hardness could be influenced by the grain boundary, dentrite morphology and dentrite arm spacing (DAS) in the alloy. At last, according to the standard deviation curves of the hardness, the chilled depths were about 5mm in 1st step and 10mm in 2nd step. And the other steps were fully chilled.