In the shearing interferometer with a single plate, the wedge of the plate is regarded in general as one of the sources of the measure errors. A new effect in the shearing interferometer with a single plate-the self-set off effect is described in this paper. Thus, the wedge of a plate isn t not only the error sources in measuring the wave front, but also at the same time obtains the radius of curvature of the wave front together with the wedge of the plate from the readings of the interference fringes counted in two different conditins.