This paper proposes a new approach to reducing the power consumption of instruction ROMs for embedded systems. The power consumption of instruction ROMs strongly depends on the switching activity of bit-lines. If a read bit-value indicates '0', the precharged bitline is discharged. In this scenario, a bit-line switching takes place and consumes power. Otherwise, the precharged bit-line level is maintained until the next access, thus no bit-line switching occurs. In our approach, the binary-patterns to be assigned to op-codes are determined based on the frequency of instructions for reducing the bit-line switching activity. Application programs are analyzed in advance, and then binary-patterns including many '1's' are assigned to the most frequently referenced instructions. In our evaluation, it is observed that the proposed approach can reduce bit-line switching by 40%.