A novel measuring method, two-period interference fringes interferometry, is described. The principle of this method is different from that of the two-wavelength interferometry that is widely known. Two interference fringes, with a little difference between their spatial periods, are obtained by turning a binary step grating and produce a synthetic equivalent period much longer than either of the two periods. The interference fringes are produced by the ±first-order beams diffracted from the grating. The intensity distribution of the interference pattern is independent of the wavelength of the laser-diode light source used. The measuring range of this method is much larger than that of the two-wavelength interferometery. A sinusoidal phase modulating technique is easily applied to detect the phase distribution of interference pattern by vibrating the grating sinusoidally. The plane reflector of ~3 mm thickness is measured to verify this novel method.