An attempt is made to formulate the finite element reanalysis to cope with the change of vibration eigenvalues and eigenvectors. The methodology is based upon iterative solution of the nonlinear, coupled equations with respect to the change of the eigen pairs. Use is made of the normalization condition of the eigenvectors. Two methods are proposed for the solution, and the first one requires less CPU time while the second is versatile enough to chase complicated change of the eigen pair. The numerical examples show the validity of the proposed reanalysis methods in regard to the vibration of portal frames which are subjected to so large structural modification in terms of the mass density and the modulus of elasticity that the eigenvector is changed markedly.