  Thermal fission energy is one of the basic parameters needed in the
calculation of antineutrino flux for reactor neutrino experiments. It is useful
to improve the precision of the thermal fission energy calculation for current
and future reactor neutrino experiments, which are aimed at more precise
determination of neutrino oscillation parameters. In this article, we give new
values for thermal fission energies of some common thermal reactor fuel
isotopes, with improvements on three aspects. One is more recent input data
acquired from updated nuclear databases. the second one is a consideration of
the production yields of fission fragments from both thermal and fast incident
neutrons for each of the four main fuel isotopes. The last one is more
carefully calculation of the average energy taken away by antineutrinos in
thermal fission with the comparison of antineutrino spectrum from different
models. The change in calculated antineutrino flux due to the new values of
thermal fission energy is about 0.32%, and the uncertainties of the new values
are about 50% smaller.
