The application of cement-treated soil as a backfill material is becoming more common in Japan. The large shear strength of treated soil can reduce the earth pressure acting on the wall. In the present study, a series of dynamic shaking model tests using a centrifuge facility was performed to investigate the seismic characteristics of the sheet pile quay wall backfilled with the treated soil. In the case of subjection to horizontal seismic force the characteristics of vibration and earth pressure of treated soil are so complicated that the improvement effects of treated soil have not been clarified yet. In addition, the treated soil is anticipated to fail with tensile cracking due to its brittle characteristics, because the treated soil cannot follow any large deformation of the surrounding soft ground. The model tests revealed the failure mode of improved ground and the effects on the deformation behaviour of a crack and dimensions of cement-treated soil.