  We study the effect of the relative velocity between the dark matter (DM) and
the baryon on the 21cm forest signals. The DM-baryon relative velocity arises
due to their different evolutions before the baryon-photon decoupling epoch and
it gives an additional anisotropic pressure that can suppress the perturbation
growth. It is intriguing that the scales $k\sim {\cal O}(10\sim
10^3)h/\mathrm{Mpc}$ at which the matter power spectrum is affected by such a
streaming velocity turns out to be the scale at which the 21cm forest signal is
sensitive to. We demonstrate that the 21cm absorption line abundance can
decrease by more than a factor of a few due to the small-scale matter power
spectrum suppression caused by the DM-baryon relative velocity.
