  Measuring the population of obscured quasars is one of the key issues to
understand the evolution of active galactic nuclei (AGNs). With a redshift
completeness of 99%, the X-ray sources detected in Chandra Deep Field South
(CDF-S) provide the best sample for this issue. In this letter we study the
population of obscured quasars in CDF-S by choosing the 4 -- 7 keV selected
sample, which is less biased by the intrinsic X-ray absorption. The 4 -- 7 keV
band selected samples also filter out most of the X-ray faint sources with too
few counts, for which the measurements of N_H and L_X have very large
uncertainties. Simply adopting the best-fit L_2-10keV and N_H, we find 71% (20
out of 28) of the quasars (with intrinsic L_2-10keV > 10^44 erg/s) are obscured
with N_H > 10^22 cm^-2. Taking account of the uncertainties in the measurements
of both N_H and L_X, conservative lower and upper limits of the fraction are
54% (13 out 24) and 84% (31 out 37). In Chandra Deep Field North, the number is
29%, however, this is mainly due to the redshift incompleteness. We estimate a
fraction of ~ 50% - 63% after correcting the redshift incompleteness with a
straightforward approach. Our results robustly confirm the existence of a large
population of obscured quasars.
