Abstract: How to measure the effectiveness of test criterion is an important problem for software quality assurance. Till now, no consistent measures have been achieved. In this paper, potential failure distance (pfd) is proposed as an indication of the effectiveness of test criterion. It regards the effectiveness of test criterion on particular software as the function of specification, program, fault type and criterion. It proceeds measure from the point of interaction between software and test criterion. Compared with other related work, it reflects the achieved confidence on the correctness or reliability about the software after testing. The measure provides a feasible comparison of the effectiveness of test criteria on software. The measure can help to design more testable software.