  The neutrino annihilation is one of the most promising candidates for the jet
production process of gamma-ray bursts. Although neutrino interaction rates
depend strongly on the neutrino spectrum, the estimations of annihilation rate
have been done with an assumption of the neutrino thermal spectrum based on the
presence of the neutrinospheres, in which neutrinos and matter couple strongly.
We consider the spectral change of neutrinos caused by the scattering by
infalling materials and amplification of the annihilation rate. We solve the
kinetic equation of neutrinos in spherically symmetric background flow and find
that neutrinos are successfully accelerated and partly form nonthermal
spectrum. We find that the accelerated neutrinos can significantly enhance the
annihilation rate by a factor of $\sim 10$, depending on the injection optical
depth.
