  We study the properties of the gravitational wave (GW) signals produced by
first order phase transitions during the inflation era. We show that the power
spectrum of the GW oscillates with its wave number. This signal can be observed
directly by future terrestrial and spatial gravitational wave detectors and
through the B-mode spectrum in CMB. This oscillatory feature of GW is generic
for any approximately instantaneous sources occurring during inflation and is
distinct from the GW from phase transitions after the inflation. The details of
the GW spectrum contain information about the scale of the phase transition and
the later evolution of the universe.
