  Leading finite temperature effects on the neutrino decoupling temperature in
the early Universe have been studied. We have incorporated modifications of the
dispersion relation and the phase space distribution due to the presence of
particles in the heat bath at temperature around MeV. Since both the expansion
rate of the Universe and the interaction rate of a neutrino are reduced by
finite temperature effects, it is necessary to calculate thermal corrections as
precisely as possible in order to find the net effect on the neutrino
decoupling temperature. We have performed such a calculation by using the
finite temperature field theory. It has been shown that the finite temperature
effects increase the neutrino decoupling temperature by 4.4%, the largest
contribution coming from the modification of the phase space due to the thermal
bath.
