A experimental method was provided, in which the distribution of reciprocal vectors can be easily obtained by their diffraction patterns. First, the diffraction pattern of square periodic superlattice as a reference grating was gotten. The value of the reciprocal vector according to the Fourier optics was calculated, and the scale relation with the geometric length in the pattern was built. By introducing the rectangular superlattice structure, this method was proved to be right in the periodic superlattices. Secondly, the diffraction patterns of the H-shape and Sierpinski fractal superlattice structures were realized and made a comparison with the square structure. The reciprocal vectors in two structures could be calculated based on the obtained geometric length ratio. Then by quantitative relation between the reciprocal vectors and fundamental wavelengths in quasi-phase matching processes, the harmonic wavelengths were calculated. Finally, the LiNbO3 nonlinear photonic crystals with fractal superlattice structures were fabricated experimentally. It can be gotten that the experimental quasi-phase matching harmonic wavelengths agree with the calculated ones. Especially, for Sierpinski fractal superlattice, by calculation, the effective second harmonic of 1.352 μm can be realized. And the corresponding results can be accomplished by experiments.