In this paper a novel watermarking method based on the discrete wavelet transform (DWT) is proposed. Different from previous watermarking methods, the watermark is embedded into the middle frequency coefficients by quantizing the middle frequency coefficient pair, which is defined as a pair of coefficients that are at the same location in the LH and HL bands of the DWT coefficients. This watermarking technique can successfully resist severe transformation of gray levels while it is still robust to normal attacks such as blurring, sharpening and JPEG compression. In detection, it does not need the original image to recover the watermark.