The nonlinear behavior of continuous structural systems which possess external resonances as well as internal resonances are found be exhibit interesting reponses, arising because of the exhange of energy between the coupled modes. In this paper, the undamped forced vibrations was studied on the effect of primary resonance based on the concept of normal modes. By using the concept of normal mode the stability relation between free and forced vibrations was investigated in case of small exciting force. Numerical results show that the excitation of one unstable mode has a great influence on the response of the other mode but that of one stable mode does not.