  The Coulomb barrier is in general much higher than thermal energy. Nuclear
fusion reactions occur only among few protons and nuclei with higher relative
energies than Coulomb barrier. It is the equilibrium velocity distribution of
these high-energy protons and nuclei that participates in determining the rate
of nuclear fusion reactions. In the circumstance it is inappropriate to use the
Maxwellian velocity distribution for calculating the nuclear fusion reaction
rate. We use the relativistic equilibrium velocity distribution for this
purpose. The rate based on the relativistic equilibrium velocity distribution
has a reduction factor with respect to that based on the Maxwellian
distribution, which factor depends on the temperature, reduced mass and atomic
numbers of the studied nuclear fusion reactions. This signifies much to the
solar neutrino problem.
