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Robustness against attacks is an important requirement in image watermarking. This paper presents a robust watermarking algorithm in wavelet transform domain. Firstly, original image is decomposed into its subbands using three level wavelet transform, then, significant coefficients with the same position in HL, LH and HH subbands of the last level are extracted to make a triplet. To embed a watermark bit into a triplet, the standard deviation of triplet coefficients magnitude is set to zero for zero bit or increased for a one bit. Three constant are used to increase the standard deviations of a triplet coefficients. The value of these constants affects the robustness of algorithm and the quality of image. To specify the optimal values for constants a Multi-Objective Genetic Algorithm (MOGA) is used. Experimental results reveal the high robustness of the proposed algorithm against common image processing attacks (blurring, median filter, sharpening, noise addition, ect), and print and scan distortion.