The development of a general procedure for the analytical approximation of the restoring arm in regular head waves is presented. The aim of the approximation is to obtain an analytical and accurate means for nonlinear parametric roll prediction. The model is based on a modified STF strip method developed for calculation of coupled heaving and pitching motion in vertical plane for large amplitudes. The non-linear effects due to large amplitude motion and variation of the wetted surface are taken into account through the variable hydrodynamic and restoring coefficients. To solve the coupled non-linear time depending equations of motions, the trigonometric series and perturbation method is applied. The roll motion is intended to be modelled dynamically using a single degree of freedom, whereas the coupling with nonlinear heave and pitch is taken into account by means of hydrostatic calculations with considering dynamic effects.