Starting from the nonlinear differential equation for a non-uniform ferromagnet in mean field approximation, we apply the bifurcation theory to discuss the pheno-mena of magnetic phase transition. It is shown that (1) the Curie point of the system is determined by the maximium eigenvalue of linearized equation; (2) the space varia-tion of magnetization just below the Curie point is described by the corresponding eigenfunction; (3) the temperature dependence of magnetization is represented by the parameter a(T)～(l-T:Tc)1/2. In the later parts of our paper, the application of this method is illustrated by examples of thin films and a simple model proposed for describ-ing non-uniform ferromagnet.