The growth and melting of nuclei with different radii in liquid Ni have been investigated by molecular dynamics simulation. The result shows that a critical nucleus is not stable and will either grow or melt at certain undercooling temperature. By calculating the upper limit of the growth temperature and the lower limit of the melt temperature, the critical nucleation temperatures for different nucleus radii can be determined, which has a linear relationship with the inverse of the nucleus radius, in accordance with classical nucleation theory and Gibbs-Thomson model. The Gibbs-Thomson coefficient of 1.56×10-7 K·m and the crystal-melt interfacial free energy of 0.211 J/m2 are obtained based on the molecular dynamics simulation of critical nucleus evolution for Ni system, consistent with the experimental results.