  The neutral hydrogen (HI) and its 21 cm line are promising probes to the
reionization process of the intergalactic medium (IGM). To use this probe
effectively, it is imperative to have a good understanding on how the neutral
hydrogen traces the underlying matter distribution. Here we study this problem
using semi-numerical modeling by combining the HI in the IGM and the HI from
halos during the epoch of reionization (EoR), and investigate the evolution and
the scale-dependence of the neutral fraction bias as well as the 21 cm line
bias. We find that the neutral fraction bias on large scales is negative during
reionization, and its absolute value on large scales increases during the early
stage of reionization and then decreases during the late stage. During the late
stage of reionization, there is a transition scale at which the HI bias
transits from negative on large scales to positive on small scales, and this
scale increases as the reionization proceeds to the end.
