Focal length of a lens is measured in a Lau setup with binary gratings. An integer order of Lau fringes appears in some conditions where a proper relation is satisfied between the involved separation distances of the setup elements, gratings periods, wavelength, and focal length of the test lens. Focal length of the lens is given by fitting the relevant theoretical curve with some unknown constants, including the focal length, to the gained experimental data. Precision of this measurement is of order of 1 mm for a lens with focal length of about 30 cm
