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The surface roughness and diffraction attenuation of a metal have an effect on the measurement accuracy of the ultrasonic scattering attenuation coef cient. In order to correct the scattering attenuation coef cient, the rough surface of the sample and its neighbouring couplant are assumed to be an equivalent medium layer, namely the sample is a multi-layered medium composed of a layer of the substrate medium and two layers of equivalent mediums. Based on the Lommel diffraction correction coef cient and the parameters of the equivalent medium layer, the expressions are developed for a circular planar piston transducer s sound eld in a multi-layered structure with equivalent medium layers. As a result, a correction model of the scattering attenuation coef cient is established by using the surface roughness and diffraction attenuation. AISI 304 stainless steel samples with different surface roughnesses are used to conduct the ultrasonic experiment. The results show that the attenuation coef cient without correction increases in proportion to the roughness; and the average relative error is up to 182.8% compared to the theoretical attenuation coef cient, while the average relative error is only 1.28% after correction. This indicates that the model can limit the negative effects of the roughness and the sound eld diffraction on the extraction of the scattering attenuation coef cient. Consequently, the corrected attenuation coef cient can improve the accuracy and reliability in the nondestructive evaluation of the microstructure.