A simplex watermarking on image resistant to RST (rotation, scale and translation) is proposed in this paper. It is a spatial watermarking algorithm on the condition of accurate self-registration geometrically, which insures watermark detection. To achieve the mentioned self-registration Radon transform is used. The watermarking detection is semi-blind. A novel correlation method is used in the detector. Theory and experiments demonstrate that the embedded watermarking survives many attacks especially for RST distortion