A form error characterization of a reflective-type scale grating, which is used in three-degree-of-freedom (3-DOF) encoders for position measurement of a planar motion stage, is presented. The scale grating has a micro-structured surface, the pitch of which is 1 μm in both Xand Y- direction. The periodic pattern on the scale grating generates diffracted beams when a laser beam incidents to the grating surface. The ±1st order diffracted beams from the grating contain information about the stage motions of not only X- or Y- directional in-plane displacement but also Z-directional out-of-plane displacement, and are therefore able to be utilized for multi-axis position detection. Accuracies of the position detection are mainly determined by a period deviation and a Z-directional out-of-flatness of the scale grating. The form error characterization of the grating is possible by using Fizeau interferometer, although the form error of a reference mirror in the Fizeau interferometer still remains as a measurement error in the form of the measured scale grating. In this paper, a new method was proposed to evaluate the form error characterization of the scale grating for the 3-DOF encoder, while eliminating the form error of the reference mirror in the Fizeau interferometer.