The phase separation kinetics of density and conformation in polymer melts is studied by the linearized time-dependent Ginzburg-Landau equations. In the present model of the free energy density, there are two order parameters which are the density and the conformation. A new nucleation mechanism is proposed, in which the fluctuation in density and the fluctuation in conformation are coupled by the mixed derivative term and the cross gradient term of these two parameters. These terms are important factors for the phase separation kinetics, since the density and the conformation further each other and induce the phase separation both in density and in conformation, which finally induce the nucleation in polymer melts. The structure factors for both density and conformation are calculated. The relevant small x-ray scattering and depolarized light scattering experimental results are compared to test this model. (C) 2003 American Institute of Physics.