Abstract: In order to investigate the effect of gravity on fluid flow velocity in freely-flowing pipe line and on selection of nominal diameter of such pipe line, calculation and analysis of the velocities of the fluid flow in two process pipe lines of different design schemes in field application are made by using the Bernoulli equation. Study result shows that the flow velocity can be greatly increased under the effect of gravity. Therefore, in design of a gravity-flow pipe line, the nominal diameter of the line or part of the line can be properly reduced according to height difference for reduction of initial capital cost and wear of pipe.