Abstract: A 2 degree of freedom system with cubic nonlinear stiffnesses is investigatedby use of the matching method and the method of multiple scales. The nonlinear modesare obtained according to their defiultion。The bifurcation and localization phenomena ofnonlinear modes in the system are analysed in detail.It is shown that the basic mechanismfor nonlinear mode localization is not nonlinear asvmmetries ais concluded by C。 Pierre et al.,but the properties of nonlinear system,i.e., the dependence of the frequency of...