We investigated ultrasonic attenuation, dynamic Young's modulus, Izod impact strength, and dielectric relaxation of acrylonitrile-butadiene-styrene composite polymers (ABS) in which styrene-co-butadiene rubber particles are dispersed in styrene-co-acrylonitrile random copolymer. High ultrasonic attenuation was observed around 240 K and the intensity of attenuation increased with increasing rubber content. Temperature dependence curves of dielectric loss in the frequency range from 12 Hz to 200 kHz and temperature range from 80 to 420 K exhibited four relaxation processes designated as the alpha, beta', beta, and gamma. From the relaxation map produced for the mechanical and dielectric relaxations, the ultrasonic absorption was attributed to the beta and beta' processes. It was deduced that inter-gain thermoelastic process, also contribute to the ultrasonic attenuation. Correlation was found between the ultrasonic absorption and the Izod impact strength indicating that the high impact strength of ABS is partly due to the effective absorption of impact energy through those relaxation processes.