  We include the effect of neutrino free streaming into the spectrum of relic
gravitational waves (RGWs) in the currently accelerating universe. For the
realistic case of a varying fractional neutrino energy density and a
non-vanishing derivative of mode function at the neutrino decoupling, the
integro-differential equation of RGWs is solved by a perturbation method for
the period from the neutrino decoupling to the matter-dominant stage.
Incorporating it to the analytic solution of the whole history of expansion of
the universe, the analytic solution of GRWs is obtained, evolving from the
inflation up to the current acceleration. The resulting spectrum of GRWs covers
the whole range of frequency $(10^{-19}\sim 10^{10})$Hz, and improves the
previous results. It is found that the neutrino free-streaming causes a
reduction of the spectral amplitude by $\sim 20%$ in the range $(10^{-16}\sim
10^{-10})$ Hz, and leaves the other portion of the spectrum almost unchanged.
This agrees with the earlier numerical calculations. Examination is made on the
difference between the accelerating and non-accelerating models, and our
analysis shows that the ratio of the spectral amplitude in accelerating
$\Lambda$CDM model over that in CDM model is $\sim 0.7$, and within the various
accelerating models of $\Omega_{\Lambda}> \Omega_m $ the spectral amplitude is
proportional to $ \Omega_m/\Omega_{\Lambda}$ for the whole range of frequency.
Comparison with LIGO S5 Runs Sensitivity shows that RGWs are not yet detectable
by the present LIGO, and in the future LISA may be able to detect RGWs in some
inflationary models.
