  We study the contribution of subhalos to the 21 cm forest signal. The halos
can host the substructures and including the effects of those small scale
clumps can potentially boost the 21 cm optical depth in favor of detecting the
21 cm forest signals. We estimate the boost factor representing the ratio of
the optical depth due to the subhalo contribution and that due to the host halo
alone (without subhalos). Even though the optical depth boost factor is
negligible for a small host halo with the mass of order $10^5 M_{\odot}$, the
subhalo contribution can enhance the optical depth by an order of magnitude for
a host halo of order $10^7 M_{\odot}$. The resultant 21 cm absorption line
abundance which is obtained by integrating over the halo mass range relevant
for the 21 cm forest signal can be enhanced by up to of order $10\%$ due to the
substructures. The larger boost factor for a larger host halo would be of
particular interest for the 21 cm forest detection because the the contribution
of the larger host halos to the 21 cm forest signals is smaller due to their
higher temperature and less abundance than the smaller host halos. The subhalos
hence can well help the larger host halos more important for the signal
estimation which, without considering the subhalos, may not give appreciable
contribution to 21 cm forest signals.
