Abstract: This article makes an analysis of nil the computable formulas, adopted recently in the determination of the economical diameter of pressure pipes for ydraulic turbines* and proves that the methods used are not yet perfect. On account of the working characters and concrete conditions of the hydraulic turbines a new method is therefore proposed to determine the economical diameter. The characteristics of the new method are ?as follows:1. It suggests a combined pnrnmetr “λ” and a concept of water head function H(D),2. It gives a more accurate calculation of hydraulic lossesthat can reflect the effectof tile varying diameter upon the friction coefficient.3. It takes into consideration the complex phenomenon of water hammer ns one of the influencing factors.