Cylinder-shaped Cu80Ni20 alloy melt is undercooled and solidified by the combination of the electromagnetic levitation technique and the flux treatment method. Nearly constant temperature gradient of 8-10 K/cm is realized for the cylindrical melts with different undercooling levels at the bottom ends. The experimental results reveal that with the increase of the undercooling of the melts from 35 to 220 K, the microstructures undergo transition from coarse dendrites to granular grains, unidirectional dendrites, and finally to equiaxed grains.