In this paper, the influence of the cooling rate on the solidified structure of ZK60 Mg alloy has been studied by means of Gleeble-1500D thermal simulation instrument. The result showed that the grain size and grain shape depended on the cooling rate, Primary Dendrite Arm Space ( λI ) and Secondary Dendrite Arm Space ( λII) sharply decreased with the increasing of solidifying cooling rate (v) in the range of experimental cooling rate (0.2~100K/s). When superheat was constant, the empirical formulas of the relation between λI λII and v was obtained. In addition, the dependence between micro-hardness (HV) and Secondary Dendrite Arm Space ( λII) for ZK60 casting alloy was proved to be similar with Hall-Petch formula. The empirical formula for λII-HV has been proposed.