Maximum undercooling of about 300 K of Ni95Cu5 alloys were achieved using molten glass purification and cyclic superheating methods. The effect of undercooling on the microstructures and growth velocity of Ni95Cu5 alloys was investigated. The macroscopic morphology of the solid-liquid interface was captured by high-speed camera and the dendrites growth velocity was calculated. The curve of growth velocity and undercooling showed a plateau stage at intermediate undercooling range and a sudden increase of growth velocity at critical undercooling. The microstructures of Ni95Cu5 alloy underwent two grains refinement, one occurred at low undercooling range and the other in the high undercooling range. By means of optical microscopy and scanning electron microscopy, the mechanism of two grain refinement was obviously different. The grain refinement at low undercooling originated from dendrite remelting, and the grain refinement at high undercooling came from recrystallization. The subgrains within Ni95Cu5 alloy microstructures indicated that the recovery occurred.