  We investigate the dependence of the gravitational wave spectrum from
quintessential inflation on the reheating process. We consider two extreme
reheating processes. One is the gravitational reheating by particle creation in
the expanding universe in which the beginning of the radiation dominated epoch
is delayed due to the presence of the epoch of domination of the kinetic energy
of the inflaton (kination). The other is the instant preheating considered by
Felder et al. in which the Universe becomes radiation dominated soon after the
end of inflation. We find that the spectrum of the gravitational waves at $\sim
100$ MHz is quite sensitive to the reheating process. This result is not
limited to quintessential inflation but applicable to various inflation models.
Conversely, the detection or non-detection of primordial gravitational waves at
$\sim$100 MHz would provide useful information regarding the reheating process
in inflation.
