A new method, in which the wavefronts of the zero-order and the positive and negative first-order diffracted beams from a planar scale grating in Littrow setup are analyzed by a Fizeau interferometer, is proposed to evaluate the Z-directional out-of-flatness as well as the X- and Y-directional pitch deviations of the planar scale grating over a large area. Meanwhile, the surface profile of the reference optical flat in the Fizeau interferometer can also be determined in a much simpler and more efficient approach than the commonly used liquid-flat reference and three-flat test calibration methods. Simulations are presented to verify the feasibility of the proposed method.