Diffraction is an important characteristics of the wave aspect of light, and grating diffraction is an important phenomenon of diffraction. A hemicycle diffraction speckle is found when a light irradiates the edge or the surface of a ruler, however, the mechanism of this phenomenon is still not clear. In this paper, combing theoretical and experimental methods we study the mechanism of the hemicycle diffracted speckle. It is found that the diffracted speckle is hemicycle when the gratinglike structure exists in the edge or the surface of a ruler. Using the standard binary grating model we analyze the diffraction of rulers, and found that the hemicycle diffracted speckle originates from the superposition of multigrating diffraction.