Abstract: Digital images have many intrinsic features such as large amount of data information,highly redundancy,and strong correlation among pixels and so on.A new method of image encryption based on bivariate polynomials is proposed according to the characters of images.In the process of image encryption,another digital image is used as the key,in this way the size of the image key could be much smaller than the secret image.The majority of steps of encryption and decryption just are add operations over a finite field,so the new method has highly efficiency.In addition,the method can be expanded to the video encryption field easily.