In scanning beam interference lithography (SBIL), a resist-coated sub-strate is scanned under phase-locked interfering beams to pattern large grat-ings or grids with nanometer level distortions. To achieve design goals, it is required to measure the spatial period, phase, and rotation of the interfer-ence fringes in real-time with high precision. In this paper, we report the use of a Fresnel zone plate to characterize interference fringes. With properly controlled errors, it should enable us to achieve high precision measurement of fringe characteristics for a wide range of fringe periods. The mathemati-cal model, details of the metrological procedure, and experimental results are presented.