The isothermal polymerization process of a common epoxy system (prepolymer: diglycidyl ether of bisphenol A, curing agent: diamine) has been characterized in the MHz range by ultrasonic pulse spectroscopy. The longitudinal wave velocity increased during polymerization. The profile of longitudinal wave velocity and attenuation depended on the curing condition (temperature and mixing ratio of the curing agent). In addition, velocity and attenuation showed clear effects of the main ((alpha)) relaxation, which can be observed during glass transition of the resin. This polymerization process was also observed in the GHz range by Brillouin-scattering measurements. In this frequency range, the polymerization process was also observed as the increase of velocity. However, the velocity values were much higher than those observed in the MHz range from the beginning of polymerization. By considering the glass transition temperature of the prepolymer (about -20 (degrees)C) used, this velocity difference indicates the possibility that the main ((alpha)) relaxation of prepolymer used situated between MHz and GHz ranges. To confirm this idea, the temperature dependences of the velocities were observed. The velocity difference became smaller as the temperature increased, which showed a smaller effect of relaxation at higher temperatures.