  We investigate reheating of the universe by early formation of stars and
quasars in the hierarchical clustering scheme of cold dark matter scenario,
with perturbation fluctuations normalized by the COBE data. It is found that
ionizing uv flux from OB stars with the abundance given by the standard initial
mass function is strong enough to ionize the universe from $z\approx 30$ to the
present epoch, if 1--2\% of the collapsed baryons go into stars. This lessens
significantly the CMB anisotropies at a small angular scale. Reionization also
increases the Jeans mass to $M_{\rm luminous}\approx 10^9M_\odot$ for $z\lsim
10$, which leads to a cut-off of the luminosity function of normal galaxies on
a faint side. A strong uv flux is expected at $z\approx 2-5$, and the null
result of the Gunn-Peterson test is naturally explained. Early star formation
also results in production of heavy elements, and the observational metal
abundance sets a strong constraint on the photon energy injection into the
intergalactic space.
