Control of the nucleation rate is important for the growth of high quality protein crystals; therefore, the development of improved growth methods is required. Crystal growth with an external electric field is an attractive method for control of the nucleation rate. However, mainly a decrease in nucleation rate was observed when an external electric field was applied using DC voltage. By focusing on the chemical potential of the liquid and solid, control of both the increase and the decrease in nucleation rate was successfully achieved by applying an AC electric field. Control of nucleation rate in terms of enhancement and retardation is reported for hen-egg white lysozyme crystals by imposition of an electric field with varying frequencies, which was determined by the electrical permittivity of both the hen-egg white lysozyme solution and crystals.