The undercooling behavior of 3 single crystal（SC）superalloys DD5, DD6 and WZ30 was investigated under the industrial process condition. During isothermal heating and cooling processes the liquidus temperature TL and the critical nucleation temperature TN of the alloys were measured. As the undercoolability the average critical nucleation undercooling ?TN = TL-TN was respectively determined to be 33, 36 and 42K for the 3 alloys. In the samples of alloy DD5 and DD6 with relatively low ?TN value, very fine dendritic microstructure was formed during isothermal cooling processes. In the sample of alloy WZ30 with relatively high nucleation undercooling, the solidification structure became a mixture of SC dendrite frame and grain clusters with random orientations. It appears that, the most random oriented grains were equiaxed crystal formed directly from the interdendritic residual melt, instead of dendrite arm fragment owing to remelting.