Abstract: Once the throughput, pipeline route and oil kinds are defined, the main factors, which influence the total cost of isothermal oil pipeline, are material, diameter and design pressure. On account of the kinds of pipe material are few, so this paper only analyzes the relation between the total cost with the diameter and design pressure. The model of total cost is established, on some condition the Hessian matrix of the total cost is positive, so the only optimal combination of pressure and diameter must exist which can make the total cost attain the minimum. On account of only one point exist which make the value of first derivative of total cost equal to zero, so the total cost is monotonous function of diameter or operation pressure, so the only minimum exist.