The free vibrations of cylindrical shells with rectangular cross-section are studied in the work. The precision of Rayleigh method for various admissible functions and different boundary conditions is evaluated. Using finite element method, the natural frequencies in the case of deviation of the cross-section shape from square to rectangular are examined, and the vibration frequencies and modes for the shell joined with the plate are studied for various plate thicknesses. Additionally, fundamental frequency behavior is analyzed in the case of fixed support, as one of the cross-section sides approaches zero.