The main lines of a drip irrigation system can be designed using a straight energy gradient line. This provides a very simple technique for selecting main line sizes that can be applied especially to evaluate alternative field layouts. An optimal shape of the energy gradient curve is determined as a curve a little bit below the straight energy gradient line; a curve has a sag of about l5% DELTA H at the middle section of the main line. The cost difference between using the straight energy gradient line and the optimal energy gradient line is small, and only around 2%. The developed technique can be used for any kind of topographic condition, uniform or nonuniform, up and down slopes. A straight line energy gradient line can be established when a main line profile is drawn and the available water pressure are determined. The concept can be applied for different types of pipes for sprinkler system design, for water supply pipe system, or transporting system for other fluids.