Free vibration analysis of non-cylindrical (conical, barrel, and hyperboloidal types) helical springs with rectangular cross-section is performed. In the differential equations of motion for the springs, all displacement functions and a generalized coordinate for warping are defined at the centroidal principal axes. The effect of warping deformation on the natural frequencies is first studied. Improved Riccati transfer matrix method is introduced to solve the first order ordinary differential equations with variable coefficients, which consist of 14 degrees of freedom. For the rectangular cross-section, the effects of the aspect ratio , the helix pitch angle , the number of active turns and the ratio of radii of the minimum cylinder to the maximum cylinder on the natural frequencies of the clamped–clamped conical springs are investigated. The natural frequencies and vibration modes of conical spring of rectangular cross-section with clamped-clamped boundary condition are simulated to validate the proposed method, which is also compared with a finite element model using three-dimensional solid elements (Solid 45) in ANSYS code. Numerical results reveal that the effect of warping deformation on the natural frequencies of the springs is prominent which should be taken into consideration in their free vibration analysis