Abstract: This paper summarizes the development of the study on the mixing volume when variations of the line diameter exist. And the influence of changing pipelines'diameter is analyzed. Then, according to the analysis on the formula of theory about diffusion and empirical formula of Austin-Palfrey, a new model for estimating mixing volumes arising in batch transportation in multi-product pipelines is presented. This new mathematical model involves in computing the variations of the line diameter as well as injection and/or withdrawal of products.