  The atomic CII fine-structure line is one of the brightest lines in a typical
star-forming galaxy spectrum with a luminosity ~ 0.1% to 1% of the bolometric
luminosity. It is potentially a reliable tracer of the dense gas distribution
at high redshifts and could provide an additional probe to the era of
reionization. By taking into account of the spontaneous, stimulated and
collisional emission of the CII line, we calculate the spin temperature and the
mean intensity as a function of the redshift. When averaged over a
cosmologically large volume, we find that the CII emission from ionized carbon
in individual galaxies is larger than the signal generated by carbon in the
intergalactic medium (IGM). Assuming that the CII luminosity is proportional to
the carbon mass in dark matter halos, we also compute the power spectrum of the
CII line intensity at various redshifts. In order to avoid the contamination
from CO rotational lines at low redshift when targeting a CII survey at high
redshifts, we propose the cross-correlation of CII and 21-cm line emission from
high redshifts. To explore the detectability of the CII signal from
reionization, we also evaluate the expected errors on the CII power spectrum
and CII-21 cm cross power spectrum based on the design of the future milimeter
surveys. We note that the CII-21 cm cross power spectrum contains interesting
features that captures physics during reionization, including the ionized
bubble sizes and the mean ionization fraction, which are challenging to measure
from 21-cm data alone. We propose an instrumental concept for the reionization
CII experiment targeting the frequency range of $\sim$ 200 to 300 GHz with 1, 3
and 10 meter apertures and a bolometric spectrometer array with 64 independent
spectral pixels with about 20,000 bolometers.
