Knowledge on the diffraction of ultrasonic waves by crack tips in the nondestructive testing (NDT) field of applications was used to develop a method and a device for the in situ crack size measurements in fatigue test specimens. This single device allows accurate measurements at every stage of the life of the test piece. The standard accuracy is 0.1 mm in the direction of propagation. In many cases we proved that it can reach 0.03 mm. Thanks to the possibility of measurements along the crack front, it is well suited for thick specimens. After some feasibility tests we designed a prototype ready for marketing. It has been used to study the local crack-tip behavior. We could not only trace evolution of the crack profile across the entire specimen thickness but also the crack closure length at any particular point versus increasing load. In this way we confirmed on A533 steel specimens different behaviors of fatigue cracks in the plane stress and in the plane strain regions. Crack closure, the blunting, and the retardation after an overload depend on the position along the front.