  Recent observations on the cosmic microwave background by Wilkinson Microwave
Anisotropy Probe(WMAP) strongly suggest that the reionization of the universe
took place quite early (z \sim 17). On the other hand, it has been pointed out
that the cold dark matter cosmology suffers from the substructure problem that
subgalactic halos are overproduced than the observed dwarf galaxies in the
Local Group. In this paper, as a potential mechanism to solve this problem, we
consider the feedback effects of the early reionization on the formation of
small-scale structures. For this purpose, we perform 3D radiation hydrodynamic
simulations with incorporating the radiative transfer for ionizing photons. As
a result, it is found that the early reionization is so devastating for
low-mass systems with M_{vir}\la 10^8 M_\odot or v_{circ}\la 20km/s, and almost
all gas is photo-evaporated in more than 95% of low-mass systems. Such a strong
negative feedback on the formation of low-mass galaxies may solve the
substructure problem and support the picture that Local Group dwarf galaxies
are descendants of the more massive halos that experienced and survived tidal
stripping.
