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 (lambda(I)) and Secondary Dendrite Arm Space (lambda(I)) sharply decreased with the increasing of solidifying cooling rate (v) in the range of experimental cooling rate (0.2 similar to 100K/s). When superheat was constant, the empirical formulas of the relation between lambda(I) lambda(II) and v was obtained. In addition, the dependence between micro-hardness (HV) and Secondary Dendrite Arm Space (lambda(II)) for ZK60 casting alloy was proved to be similar with Hall-Petch formula. The empirical formula for lambda(II)-HV has been proposed.