We propose a simple method of determining the surface anchoring energies of liquid crystals (LCs) in a binary grating structure. In the configuration of an alternating hybrid and homeotropic geometry, the director of the LC molecules is linearly deformed near domain boundaries according to the continuum theory. From the far-field optical diffraction formula for the phase profile in a form of a trapezoid, the extrapolation lengths, corresponding to the distorted lengths at a domain boundary, were obtained. The surface anchoring energies of the LCs are directly determined from the measured extrapolation lengths. (C) 2004 Elsevier B.V. All rights reserved.