Abstract: By statistical analysis of a lot of video sources, we present an Improved Weibull density distribution. We develop a new ratedistortion model which is based on Improved Weibull density distribution. We use Weibull probability density function to approximate actual DCT coefficients distribution. With encoding entropy theory and distortion analysis, the model is expressed as two functions of the video coder quantization step size, which are ratequantization model (R-Q) and distortion-quantization model (D-Q) that are based on the density distribution, and the model which is based on Improved Weibull density distribution is to estimate encoding rate and quantization distortion. Simulation experiments results show that the density distribution is closer to the statistical distribution of the DCT coefficients than Cauchy density distribution. The model which is based on Improved Weibull density distribution provides better estimates for the actual encoding rate and quantization distortion performance than those that are based on Cauchy density distribution.