Abstract: Contmination between batches in multi-products pipeline transport is studied.The influences of convection and diffusion on the contamination are studied in detail.Diffusion equations. which are mainly controlled by convection. are developed under turbulent pipe flow. The diffusion equation is separated into a pure convection equation and pure diffusion equation which are solved by characteristics method and finite difference method respectively to obtain numerical solutions The results of numerical computation explain the forming and developing of contamination very well.