In this paper we have given the quantitative expression of the moire fringes of circular Concentric gratings in the fild of measuring the horizontality of flat surface and the experimental results. In measuring the horizontality of flat surface by the moire frings of circular concentric gratings. we can give the results directly. not to rotate the gratings or the flat surface. The experiments have demonstrated that there isgreat advantage in measuring the horizontality of flat surface by the moire frinqes of circular concentric gratings.