Nuclear reactors generate a highly intense flux of electron-antineutrinos from fission products through β- decay, and produce a slight number of electron-neutrinos through either β+ decay or electron capture. Neutrino energy spectra are usually calculated by the β decay theory. Since the reactor neutrinos are emitted from a great number of fission products, the calculation requires many level schemes of these nuclides. Nuclear data files, however, are available these days. It is possible to evaluate the electron-antineutrino and -neutrino spectra for a nuclear reactor on the basis of nuclear data files (JENDL-FP-Decay-Data-File-2000, JENDL-3.3). In this study, we derive electron-neutrino and -antineutrino spectra in the energy range of 10 keV to 8 MeV from nuclear data files. The method gives good agreement with other studies for electron-antineutrino spectra. We show a simple method for estimating the reactor neutrino spectra without complicated computation.