Abstract: A key issue brought by partial enhanced-scan delay testing is how to select the critical normal-scan flip-flops to be replaced with enhanced-scan cells.We present a flip-flop selection method based on the defined relative measure between a normal-scan cell and the undetected transition delay faults,by which a small number of selected normal-scan cells can be replaced with enhanced-scan cells to improve the probability of detecting transition delay faults effectively.Experimental results demonstrated that the proposed method can improve transition delay fault coverage with acceptable hardware overhead.