Shearography is a whole-field optical technique used to measure the surface displacement derivatives. The digital shearing speckle interferometer (DSSI) allows the acquisition and observation of fringes in real time without having to undergo the photographic wet-process and the filtering procedure. In this paper, the principles of DSSI are introduced. In this method, the undeformed speckle image is firstly digitized and stored into the computer through a CCD camera. The subsequent frames of the deformed object are then subtracted continuously from the stored reference frame, thus a fringe pattern is formed. By counting the fringe order, the displacement derivatives at any point on the surface of the object can be determined. A case study using DSSI method to measure the derivatives of surface displacements is presented.