High speed wavefront sensing is important in real time profile analysis, analysis of fluid dynamics, ophthalmology and so on. Conventional Shack-Hartmann wavefront sensor uses an array of tiny lenses and a digital camera to record the focal spot array. Thus the frame rate of the sensor depends on the camera. In this paper we present a zonal wavefront sensor where the array of lenses is replaced by an array of gratings followed by a focusing lens. The gratings can be configured to generate just one array of focal spots. This reduction in row of the focal spot array leads to increase in the frame rate of the proposed wavefront sensor.