Abstract: Low efficiency is a main problem when the RISC microprocessors implement S-box substitution operation.In order to attack it,an approach is proposed in this paper,which is done by extending instructions through designing special module and instruction for S-box substitution,thus raising the performance of it.A new kind of Sbox substitution module and operation instruction is designed based on the characteristics of S-box substitution operation of block cipher algorithm,which can support the four common modes of S-box substitution module,i.e.,8-32,8-8,6-4,4-4mode.The circuit implementing S-box substitution module is then designed adopting the Boolean function method.The concept is finally verified in EDA software,which shows the validity of the method and reveals the significant improved performance.