The energy modal synthesis method is applied to the computation of the neutron-multiplication factor, the sodium-void coefficient, and the spatial-power distribution of an idealized fast reactor configuration. The method is formulated by means of the variational principle and the error estimator is introduced by the expansion technique of the trial functions in the eigenfunction series. In the space-independent case, the sodium-void coefficient and its error are obtained very accurately by the linear combination of two modes, and the optimal two modes are chosen by the physical consideration. In the one-dimensional case, the continuous-type and the discontinuous-type energy modal synthesis method are formulated. In both cases the infinite-medium spectra and some supplementary modes are used as the trial energy modes. The results of a few-mode synthesis give a good estimate of the reactor parameters, and the error can be evaluated successfully by the error estimator.