Abstract: The effects of internal hydrostatic pressure on input vibration power flow in cylindrical shells filled with fluid were investigated. The hydrostatic pressure was modeled using static pre-stress terms in the shell eq uations of motion. The structural and fluid equations of motion were taken from Fltigge thin shell theory and Helmhohz wave equation respectively. By using the Fourier transform and its inverse transform and applying r esidues theorem, the input power flow into the coupled system under a line circumferential cosine harmonic driving force was studied. The results showed that the hydrostatic pressure had practically no effect on inpu t power flow for circumferential mode order n=0. For high circumferential modes, the input power flow curves translated to right along frequency axis when considering hydrostatic pressure at mid and low frequencies. Si nce the frequencies of the peaks in the input power curve corresponded to the cut-on frequencies of the propagating waves in the dispersion curves, the cut-on frequencies of the corresponding propagating waves incr eased. For higher pressure and larger circumferential mode order, the effects input power flow was more significant.