In this paper, a new self-testing method for combinational circuits backed up with polymorphic gates is presented. Testing feature is completely merged with normal circuit operation where the single stuck at fault model is applied. Experimental results show that our method is able to detect all stuck-at-faults with reduced number of test vectors and insignificant amount of redundancy and high fault detection probability compared to other methods. Also, the proposed approach can be scaled up to utilize in testing of not only small scale, but also large scale circuits.