In this paper, we propsoe an encryption algorithm of image information using a mass transfer function (MTF). The algorithm is based on a diffusion phenomenon of black ink when black ink dropped in the stationary water. We mathematically analyze the phenomenon, in consideration of characteristics of image information, and apply the results of analysis to the security of image transfer phenomenon. The cryptosystem proposed in this paper enables the security services of information in narrow-band channel communication network to be provided. And in transmission of important information, it can secure against intentional disturbance and violation. Also, it can guarantee the safe flow of information.