We propose an experimental method for measuring the period difference in a grating pair. The method uses the relationship between the period difference and the phase difference of diffraction beams of the grating pair when gratings are translated along the grating-vector direction. The effect of the roll angular deviation of gratings can be eliminated by phase compensation with $+1$ st- and $-1$ st-order diffraction beams. Far-field intensity patterns of diffraction beams are monitored to adjust the relative attitude of gratings. For a pair of gratings with periods of $\sim 0.674\mathrm{\mu m}$ , the period difference was measured to be $0.4434\mathrm{nm}$ with an expanded uncertainty ( $k=2$ ) of $0.0113\mathrm{nm}$ .