A direct-current potential-drop NDE technique has been developed for the evaluation of a 3-D back-wall crack in thick-walled structures. The measuring system contains two pairs of probes – one for current supply and the other for the measurement of potential-drop, where the currentsupply and measuring probes are in close proximity to each other. The optimum arrangement of the four-point-probes system is determined from the finite-element simulation of the three-dimensional electric crack problem. The predicted potential-drop across the crack has been verified experimentally using the newly developed measuring sensor. Measured results of semi-elliptical cracks in stainless steel plates establish the practicality as well as reliability of the present potentialdrop method of testing for the evaluation of 3-D back-wall cracks in thick-walled structures.