In this paper, a wavefront curvature sensor is presented. This sensor is based on the measurements of the differentials of wavefront slopes, where the wavefront slope measurements can be achieved by a Shack-Hartmann sensor. A Shack-Hartmann sensor with three output collimated beams will be introduced with a lenslet array installed in each beam. By shifting two of the Shack-Hartmann grids in the x- and in the y- directions independently compared to the third grid, we can measure the slope differences and obtain the wavefront curvatures at each grid point. A wavefront reconstruction can be performed with the wavefront curvature measurements.