A simple method for the measurement of angular displacement is proposed. As a laser beam is incident on a planar optical waveguide, the m-lines will be obtained by scanning the angle of incidence. It is found that the m-lines will shift with the variation of the thickness of the guided layer. And interesting measurement approaches which are based on intensity measurement and angle interrogation, respectively, are described. Theoretical results and simulation show that intensity measurement is more sensitive than angle interrogation. And small angle of incidence is more sensitive than large angle of incidence which corresponds to high-order modes and low-order modes, respectively.