Microstructures of steel are the major variable which determines mechanical properties such as strength, toughness and fatigue limit. Ultrasonic wave propagating in polycrystalline metals is mainly attenuated by grain boundary scattering. Effect of ultrasonic attenuation on microstructure changes in plain carbon steel consisting of phases such as ferrite+pearlite, ferrite+bainite, bainite and martensite was evaluated. Ferrite+pearlite steel yielded greater attenuation than bainitic steel, and bainitic steel showed higher attenuation than martensitic steel. From analysis of relationship between attenuation and microstructure comprising grain and subgrain boundaries, it was confirmed that the main scatterer of martensite structure could be packet boundary. This result shows that microstructures of steel could be identified nondestructively by the measurement of ultrasonic attenuation.