The microstructures of nickel solidified at different cooling rates are studied by using molecular dynamics simulation and the critical condition for nickel melt to form ideal metallic glass is calculated. The simulation results show that the crystal structure is obtained after the nickel melt has been solidified at a cooling rate that is lower than 1011 K/s; while a mixture is composed of crystal structure and amorphous structure when the cooling rate is in a region from 1011 K/s to 1014.5 K/s. The solidified crystal of nickel is of fcc structure when the cooling rate is lower than 1010 K/s, while it changes into crystal structure composed of fcc and hcp when the cooling rate is between 1010 K/s and 1014.5 K/s. By analyzing the calculation and simulation results, it is determined that the critical cooling rate for nickel melt to form ideal metallic glass is 1014.5 K/s. Moreover, it is found that the structures of the subcritical nuclei (the cooling rate is higher than 1014.5 K/s), critical nuclei (the cooling rate is 1014.5 K/s), and the growing crystal (the cooling rate is lower than 1014.5 K/s) are the lamellar structures composed of fcc and hcp atoms, which indicates that the subcritical nuclei, critical nuclei and the growing crystal have the same structures.