In order to nondestructively measure the domain structure of periodically polarized crystal, the rigorous coupled-wave analysis was used to analysis the period and duty cycle of the microstructure. A laser with a wavelength of 532 nm and power of 20 mW was used as the light source. The beam with TE polarization is vertically incident on the periodically poled lithium niobate crystal surface, at the same time the voltage is applied to the crystal to increase the refractive index difference between the positive and negative domains. The power of each diffraction orders is detected by a photo detector . The period and duty cycle of the periodically poled lithium niobate crystal can be got by using the least squares method to fit the simulation results of the rigorous coupled-wave analysis and the experimental data. The experimental accuracy of the period and the duty cycle is 0.05 μm and 0.05 respectively. Compared with the period calculated by the Prague diffraction formula, the precision is improved by an order of magnitude.