In the shadow-moiré system, the period of the grating is varied by rotation of the grating, so the phase of the moiré pattern is changed as well. By the selection of suitable rotation angles, three images at different positions of the grating are acquired to obtain the absolute distance from the object to the grating. A theoretical analysis is presented for the method, and some experiments have been done to verify the theoretical analysis. The results show that the method is fast and the accuracy is better than 10 μm. The measurable range is directly proportional to the period of the grating and inversely proportional to the angles at which the grating is rotated.