A device of applying protection bit codes to encrypt a program for protection is disclosed. The program has a plurality of instructions P (positive integer). The device includes a protection-bit-code generator, a first protection-bit-code location generator and a protection-bit-code insertion unit. The protection-bit-code generator generates a plurality of protection bit codes in accordance with the plurality of instructions of the program, wherein each instruction has a plurality of bits I (positive integer). The first protection-bit-code location generator generates a plurality of insertion positions N (positive integer) for each protection bit code in accordance with processor status when executing the program. The protection-bit-code insertion unit inserts each protection bit code in a position between (N-1)th- and Nth-bit of a respective instruction of the program in accordance with the insertion position N generated by the first protection-bit-code location generator, thereby generating an encrypted program.