Abstract: This study presents the method to study the free and forced dynamic response of a shaft of a Pelton turbine unit. Equation of motion for the bending vibration of the shat of the Pelton turbine assembly is developed by using Hamilton principle by assuming Pelton wheel as a rigid disk attached on a Timoshenko shaft. Impact provided by the water jet is represented by Fourier series expansion. Governing equations of the system in terms of coupled partial differential equations are converted into coupled modal equations by using assumed mode method. Then, these coupled ordinary differential equations are solved for free and forced response of the system.