• Investigation of dynamics of closed end hydraulic transmission lines has been carried out in order to develop a simple mathematical model Capable of representing these lines accurately. The partial differential equations governing the dynamics of the lines have been solved for harmonic excitations under the assumption of constant linear frictional resistance, or a linear frictional resistance and alternatively by solving, Navier -Stokes’ equations. Comparing the theoretical predictions with experimental results showed that the first treatment fails to predict the dynamics of the lines accurately, while the second and third treatments yield good agreement with the experiments. A simplified mathematical model for a transmission line can be obtained by representing the line with lumped parameters, which might be of different arrangements and various numbers. Theoretical prediction of the line frequency response showed good agreement with the measured responses when the frictional resistance of the line is assumed to be linear and varies with the exciting frequency, and the number of lumps equals four. Each lump should consist of one fourth the line frictional resistance, inertial resistance, and capacitance, and according to the mentioned arrangement.