Basing on Young′s double-slit interference, the grating′s Talbot and fractional Talbot phenomena under illumination of parallel homochromatic wave were discussed. The grating diffraction fringes on the imaging plane results virtually from the so-called erasable coherent superposition of many Young′s interference fringes of double-slit with certain distance within the grating. Although the distance between fringes and the places of bright and dark fringe were not unvaried, there would be a shift of half period of the image versus the grating viz. fringe distribution versus scratches of grating. This shift would be determined by parity of parameter α of double-slit, namely that when α is even, the shift don′t appear, while α is odd, it appear.