The higher order perturbation theory is applied to the one-dimensional multi-group diffusion equation. The method of calculating the eigenvalue, eigenfunction and adjoint function of the higher mode is established for a non-Hermetian matrix. The diffusion equation is solved by the higher order perturbation method, through the use of the eigenvalues, eigenfunctions and adjoint functions of the higher modes. The newly developed method is applied to the estimation of reactivity effect in a reactor.