Superparamagnetic nanoparticles are used as an imaging agent. Magnetic particle imaging (MPI) is applicable to not only diagnostics but therapy such as hyperthermia and drug delivery. Integrative therapeutic and diagnostic application is called as theranostics. In particular, magnetic relaxation is one of the most important factors for both imaging and hyperthermia treatment. The evaluation of magnetic relaxation property is necessary to design the materials and instrument applying an external magnetic field. Magnetic relaxation is characterized by two distinct modes. Brownian relaxation is occurred by rotation of the particles. Néel relaxation is induced by rotation of the magnetic moment. The mechanism of magnetic relaxation is influenced by the primary and secondary size of magnetic nanoparticles, surface coating agent, state of materials, viscosity of medium. In this study, the magnetic relaxation properties of Resovist and other commercial magnetic nanoparticles were measured at frequency of several kiro heltz up to 500 kHz and high magnetic field up to 50 Oe. The commercial iron oxide nanoparticles of both of superparamagnetic and ferrimagnetic features were used for the measurements. The hysteresis loop under an applied alternating magnetic field was measured for estimating magnetic loss and effective magnetic relaxation time. The materials were prepared in powder form, dispersed state in medium, or fixed state by agarose / epoxy bond. Magnetic nanoparticles dispersed in medium were rotated by an alternating magnetic field, and both Brownian and Néel relaxation could occur. In contrast, fixation regulated rotation of the particles and only Néel relaxation could occur. Therefore, hysteresis loop of alternating magnetic field in dispersed and fixed samples directly indicated the magnetic relaxation mechanisms of measured materials. The dependency of area of the hysteresis loop on the frequency of magnetic field indicated the effective frequency of Brownian and Néel relaxation. Relaxation time was also calculated. Experimental estimation of effective relaxation time confirms the influence of size distribution, concentration, state, surface coating agent and medium. Thus, evaluation of magnetic loss and magnetic relaxation time measured under the alternating field condition which is used for MPI is the effective method to evaluate the mechanisms of magnetic relaxation and optimum material property.