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The quality of watermark mainly depends on scaling factor, i.e., the ratio in which we mix the host image and watermark image. This scaling factor should not be too low as it will degrade the quality of extracted watermark (robustness) hugely. Similarly, it should not be too high as it will degrade the quality of watermarked image (Imperceptibility) hugely. So there is a need of optimal selection of scaling factor to get a trade-off between robustness and imperceptibility. In this work, particle swarm optimization (PSO) has been used to choose the optimal values of the scaling factors. Singular value decomposition (SVD) has been used for the watermarking because of its high capacity. Five different attacks have been considered on the watermarked image. The simulation result shows that the SVD + PSO-based watermarking outperforms the watermarking based on SVD alone in all the five attacks.