  Recent cosmological observations indicate the existence of extra light
species, i.e., dark radiation. In this paper we show that signatures of the
dark radiation are imprinted in the spectrum of inflationary gravitational
waves. If the dark radiation is produced by the decay of a massive particle,
high frequency mode of the gravitational waves are suppressed. In addition, due
to the effect of the anisotropic stress caused by the dark radiation, a dip in
the gravitational wave spectrum may show up at the frequency which enters the
horizon at the time of the dark radiation production. Once the gravitational
wave spectrum is experimentally studied in detail, we can infer the information
on how and when the dark radiation was produced in the Universe.
