The undercoolings of Ni-coating melt with different temperatures were in situ tested within differential scanning calorimeter facility by a flux technique. The highest undercooling obtained in this way was 411 K. The undercooling increased with the increasing of cooling rate in rang of 10 K/min-50 K/rain and melt treatment temperature at the same cooling rate, then held a constant. The relationships between crystallize rate and temperature as well as cooling rate and time were analyzed. It shows that a faster cooling rate results in a shorter time needed to finish the melt crystallization.