The characteristics of soft clays are very important for the land development plan. This study is to investigate correlations between the engineering properties and the characteristics of clay minerals of the undisturbed clay samples obtained from Sihwa area. This study included X-Ray diffraction analysis, X-Ray fluorescence spectrometer analysis, scanning electron microscopy analysis and energy dispersive X-Ray spectrometer analysis. The correlations between the clay mineral properties and the laboratory and field testing results were investigated. The characteristics of soft clay in Sihwa area were compared with those in Yangsan and Kunsan area.