Abstract: Quantization Indexing Modulation (QIM) method is usually used in watermarking and information hiding．Quantization step is a key parameter of quantization modulation method．Its value connects with watermark embedding degree．Now there is no theoretical method to decide its value except through experiments,and this will reduce watermark inserting efficiency．To overcome this difficulty,a quantitative estimation method of quantization error for Dither-quantization Modulation was provided based on its distribution at first．Then based on the estimation method,selecting spatial pixels as quantization coefficients,the quantitative relationship equation of quantization step,watermark sequence length and PSNR was deduced．Experiments results show PSNR values calculated through quantitative equation are in good agreement with those obtained from experiments under equal conditions．It demonstrates that the deduced quantitative relationship equation is accurate.

Key words: Quantization indexing modulation(QIM),Spatial domain,Peak signal-to-noise ratio (PSNR),Quantization step,Quantization error,Quantization coefficients