Experimental and theoretical results of investigation of an asymmetrical magneto-optic diffraction by a domain structure with oblique domain walls in birefringence crystals are here presented. It has been found that the inequality of intensities and polarizations of light in the first diffraction orders are caused by the asymmetrical changing of the light wave phase over the domain structure period. It is shown that a plate with such a domain structure can be considered as a magnetic phase grating with four elements in the period.