With the development of manufacturing industry, the high precision size measurement for the holes on the surface of mechanical parts is required. A method for measuring size of the hole based on double projection is proposed in this paper. The CCD camera separately collects grating images which projected onto the part surface in two directions. The phase-shift method is used to calculate the phase of grating images in two directions, and the reconstructed three-dimensional shapes in the two directions are merged to measure the hole dimension. Experiment results show that the measurement accuracy of the thickness is 0.04mm, and the hole size can reach to 0.1 mm.