Random-phase marks in the Fresnel domain to scramble optical image are proposed.The system needn't any lens and image encryption in the system is completely achieved by Fresnel diffraction.The method can encrypt an original image to random noise by using two statistically independent random-phase masks put respectively in the input and transformed planes.The wavelength of monochrome plane wave,the distance of propagation and the phase codes,are used as secret code keys to encrypt the original image.The system is simplified and higher security is achieved.