The paper shows the impact of qubic non-linearity of spring force on the vibrations of the Duffing oscillator. The comparison leads to the conclusion that, nonlinearity can play a significant role both in case of free and forced vibrations. A number of characteristic phenomena have been observed, i.e. bending resonance peak, and influence of non-linearity on the natural frequency. Received results shows clearly that non-linearity should not be neglected if accurate models are required.