This article solves the task of determining frequencies and modes of free oscillations of trunk thin-wall oil pipelines of big diameter in case of semi-underground laying. The task will be solved on the basis of geometrically non-linear variant of semi-moment theory of cylindrical shells of moderate flexure. The resolving equations have been acquired, which make it possible to investigate the frequencies of free oscillations of oil pipeline depending on the physical and geometrical parameters in the process of operation. The oil pipeline is exposed to the effect of stationary internal working pressure, parameter of longitudinal compressive force, influence of soil reactive pressure at different geometrical characteristics as well as influence of fluid flow hydrodynamic pressure on the pipe internal wall. The fluid hydrodynamic pressure has been determined on the basis of theory of potential flow of ideal incompressible fluid in cylindrical coordinates with the use of modified Bessel functions of the first genus. The influence of soil surrounding the pipe corresponds to a non-uniform load caused by the bearing pressure of soil; the value of bearing pressure will depend on soils under consideration.