Brightness-and-contrast adjustment is the most common image processing ,while lots of existing image watermarking algorithms in discrete consine transf orm (DCT) domain cannot resi st brightness-and-contrast adjusting attack.Therefore,a revision formulas fo r resisting brightness-and-contrast adjusting attack is proposed,based upon the method of the revision for resisting brightness adjustment and the essential ana lysis on the method of relative quantization.According to the proposed revision formulas,this paper proposes a novel image digital wate rmarking algorithm in DCT domain for resisting brightness-and-contrast adjusti ng attack.Experimental results show that the proposed algorithm has good trans par ency for the embedded watermark and is strongly robust against brightness-and- co ntrast adjusting attack,and the algorithm is also robust against other common i mage-processing attacks,such as Gaussian filtering,median filtering,average filtering,resampling and JPEG compression.