This study presents an image watermarking scheme that uses optimization-based mean quantization in the wavelet domain. In the proposed scheme, multi-coefficients of DWT are utilized for image watermarking. To modify these coefficients in mean quantization technique, an optimization formula is derived. First, the peak signal-to-noise ratio (PSNR) is expressed as a performance index in matrix form. Then, an optimized-quality functional that relates the performance index to the mean-quantization technique is obtained. Finally, the Lagrange Principle is utilized to obtain the optimal solution. The optimal solution is applied to watermarking. Experimental results show that the watermarked image can keep high PSNR and achieve better BER even when the number of coefficients for embedding a watermark bit increases.