A blind watermarking method by grouping codewords for VQ-quantized images is disclosed. Especially there is provided a watermark insertion method in which a codebook is divided into three groups satisfying specific standards and a codeword is allocated based on the group to which the corresponding codeword belongs to insert watermark information, and a watermark extraction method for extracting the watermark inserted by the watermark insertion method. According to the present invention, watermark information can be uniformly inserted into an input vector space and the original image is not needed for extracting the watermark information.