In order to utilize mass of oyster shells for a partial substitute material for reclamation, we investigate the undrained shear characteristics of dredged sludge mixed with oyster shells by tri-axial compression test and numerical analysis based upon the mathematical homogenization. The following conclusions can be obtained from this study. It is verified that the shear strength of dredged clay is improved by mixing with oyster shells. The increase of the undrained shear strength is almost proportional for mixing rate (0-80%) of the oyster shell, and it is independent of consolidation stress (49-98kPa). And, it was guessed that the structure of the oyster shell in mixed soil complicatedly changed from stress-strain relationship and pore water pressure generation characteristic in proportion to the consolidation stress, oyster shell mixing rate, and the shear strain level.