This paper presents a nonwatermarking algorithm based on the most significant bits to construct the key. Firstly, the binary watermarking image is scrambled and extended to the size of the main image.Then the value of every pixel of the watermarking image is compared with that of the corresponding MSB (Most Significant Bit) of the main image. When the compared value is true,the location of the main image is marked by a zero matrix, and the marked zero matrix is a key that can be used to detect the watermarking image later. Good performance can be achieved due to the watermarking information which is relatively in the most high bitplane and is scattered to the whole of a main image. The experimental results show the validity of the method.