A new moiré interferometer for determining 3-D displacement field of a slightly strained body is proposed. In this interferometer, the specimen grating not only diffracts the incoming beam into several beams but also rediffracts the diffracted beams, each of them is reflected from a mirror back to the path of the incoming beam. The rediffracted beams are then directed to a camera to generate three interference patterns, which are sufficient to determine the 3-D displacement field. The governing equations corresponding to these interference patterns are derived, and the advantages of using this interferometer are described. Some experiments were carried out to verify the governing equations and advantages.