The longitudinal ultrasonic velocity, longitudinal ultrasonic absorption (attenuation coefficient), glass-transition temperature, and Mooney viscosity for epoxidized natural rubber/ethylene-propylene diene monomer blends were measured. The variation of the longitudinal ultrasonic velocity with the blend ratios was linear, indicating a compatible system in comparison with the same system without epoxidation (natural rubber/ethylene-propylene diene monomer), which was incompatible. Also, the behavior was confirmed by heat of mixing calculations as well as Mooney viscosity measurements.