A microscopic mechanism to describe the magnetic field H dependence of the polarization P is proposed on the basis of a microscopic model for RCrO3 including the spin-phonon interaction. Using the Green’s function theory in Peierls systems we have studied the two contributions to P due to the antisymmetric exchange Dzyaloshinskii-Moriya interaction P AS and the magnetostriction P MS . The behaviour of P(H) – increase or decrease – in different multiferroics can be explained in our model taking into account the second order spin-phonon interaction constants. When a magnetic field is applied to a magnetoelectric material, the material is strained. H changes the average of the spin operator products and so P. Moreover, H renormalizes the elastic constant which explains the strain effect. From here we have found a qualitative relationship between pressure and magnetic field effects on P. The observed results are in good qualitatively agreement with the experimental data.