More than 50 undisturbed and remoulded clayey soil specimens were tested using hollow cylindrical torsional testing apparatus. The marine clay was sampled from 5 different harbours and coastal locations in western Japan. Sand and clay were mixed in various proportions for testing. The plasticity index of the sampled soils varied from 16 to 111 and the particle distribution curves were also variable. Based on experimental results, a unified formula representing the dynamic shear moduli and damping ratios for the clayey soils with various plasticity and sand contents is presented.