[en] In the conventional perturbation treatment by using the partitioning techniques, the references ket is chosen to be an eigenket of the unperturbed Hamiltonian H sub(o), or a finite sum of eigenkets of H sub(o). A modified perturbation theory where the reference ket is a true perturbed ket, that is, an eigenket of H = H sub(o) + V is presented. A perturbative expansion is generated from an exact operator equation and an explicit solution obtained by successive approximations treatment. The third - and higher order terms in the new expansion are different from the analogous terms in the usual perturbation theories. (Author)