In the study, a temperature acquisition system was used to measure average solidification cooling rate of GE81 alloys under different cooling conditions, such as furnace-cooled in graphite mould, air-cooled in graphite mould, air-cooled in steel mould and air-cooled in copper mould. The connection between cooling rate and grain density was studied on the basis of classical nucleation theory. The optical microscope (OM) and scanning electron microscope (SEM) were used to observe the microstructure of as-cast GE81 alloys (Mg-8%Gd-1%Er, mass fraction) under various cooling rate in order to establish the relationship between solidification cooling rate, grain density, hardness and volume fraction of secondary phases. The results showed that the solidification cooling rate of GE81 alloy in the four moulds was 0.23, 0.46 2.17 and 3.88 K·s-1, respectively. The cooling rate is linear with the undercooling and the formula was ?T=13.5664v+6.9655. With the increasing cooling rate, the α-Mg grains were refined obviously. The relationship between the cooling rate and the grain density could be described as Nv=1.1135×1012exp(-46.8344/(13.5664v+6.9655)). Moreover, the secondary phases distributed more homogeneously and their volume fraction decreased so that the hardness of GE81 alloy increased significantly. The relationship between the hardness and the cooling rate could be expressed as HV=72.1772-12.6895/(1+exp(v-2.2570)).