A new approach is proposed to translate existing software programs from one instruction set other instruction sets at the assembly level. The behaviors of instructions are abstractly represented as manipulation of the machine state. The behavior of each basic block of the software program is then represented as state transition pair. Instruction set retargeting is then modeled as the problem of finding sequences of instructions accomplishing the same machine state transitions at the target machine as does the software program at the source machine. The proposed approach has been successfully demonstrated for software translation between several industrial microcontrollers.