Magnetic nanoparticles (MNPs) have been widely studied for use in biomedical and industrial applications. The frequency dependence of the magnetization of magnetic nanoparticles is analyzed for different AC excitation fields. We employ a Fokker-Planck equation, which accurately describes AC magnetization dynamics and analyze the difference in AC susceptibility between Fokker-Planck equation and Debye model. Based on these results we proposed a simple, empirical AC susceptibility model. Simulation and experimental results showed that the proposed empirical model accurately describes AC susceptibility, and the AC susceptibility constructed with the proposed empirical equation based on Debye model agrees well with the measured results. Therefore, we can utilize the proposed empirical model in biomedical applications, such as the estimation of the hydrodynamic size and temperature, which is expected to apply to biologicals assays and hyperthermia.