Solidification cracking of Mg welds has been reported frequently, but the crack susceptibility itself has not been studied much. In the present investigation the widely used Mg alloys AZ31, AZ61, AZ91 and ZK61 were selected for the study. The crack susceptibility was predicted based on the maximum │dT/d(f_S)^1/2│ up to (f_S)^1/2 = 0.99 as the crack susceptibility index (T: temperature; f_S: fraction of solid). The predicted crack susceptibility decreased in the order of ZK61 > AZ31 > AZ61 > AZ91. Since no reported data were available for comparison with the prediction, the transverse motion weldability (TMW) test was conducted. The tested crack susceptibility decreased in the order of ZK61 > AZ31 > AZ61 > AZ91, thus verifying the prediction based on the index │dT/d(f_S)^1/2│. The present study demonstrated that the crack susceptibility index and the TMW test can be useful tools for studying solidification cracking of Mg welds.