We investigate the isothermal crystallizations of nanoparticles composed, respectively, of 4000 Cu atoms (Cu4000) and 13500 Cu atoms (Cu13500), and bulk Cu according to on embedded atom model, using molecular dynamics simulations. We note that different sizes of Cu nanoparticles display multistep crystallization at low temperature, and their crystallization time distribution is wider than at high temperature, shown by analyzing the structural and dynamic properties of isothermal crystallization. Moreover, the size of particle plays an important role in the crystallization process. The larger the size, the longer the crystallization time is. However, we find that there is a critical size rc. The crystallization time increases with particle size increasing when the size is less than rc. On the contrary, when the size is more than rc, the crystallization time decreases with particle size increasing.