Sand compaction pile (SCP) has been widely utilized for the improvement of the bearing capacity and settlement of soft ground. Although the static stability mechanisms of SCP have been well understood, little has been known about the details of the dynamic properties. As a result, the seismic design standard ignores the differences of dynamic properties between improved and unimproved ground.
　This study aims at clarifying the dynamic characteristics of SCP-improved ground in order to provide more reasonable design procedures. A series of shake tests was undertaken at different improvement ratios under centrifuge accelelration of 50 G. The dynamic properties of each case were evaluated by transfer function and relative displacements which were calculated from the measured accelerations.
　It was found that dynamic properties of SCP improved ground were between the values for the original soft clay ground and dense sand ground. The results also revealed that, as the improvement ratio increases, the dynamic characteristics of SCP improved ground and dense sand ground became closer.