  The charged Higgs boson is quite common in many new physics models. In this
study we examine the potential of observing a heavy charged Higgs boson in its
decay mode of top-quark and bottom-quark in the Type-II
Two-Higgs-Doublet-Model. In this model, the chirality structure of the coupling
of charged Higgs boson to the top- and bottom-quark is very sensitive to the
value of $\tan\beta$. As the polarization of the top-quark can be measured
experimentally from the top-quark decay products, one could make use of the
top-quark polarization to determine the value of $\tan\beta$. We preform a
detailed analysis of measuring top-quark polarization in the production
channels $gb\to tH^-$ and $g\bar{b}\to \bar{t}H^+$. We calculate the helicity
amplitudes of the charged Higgs boson production and decay.Our calculation
shows that the top-quark from the charged Higgs boson decay provides a good
probe for measuring $\tan\beta$, especially for the intermediate $\tan\beta$
region. On the contrary, the top-quark produced in association with the charged
Higgs boson cannot be used to measure $\tan\beta$ because its polarization is
highly contaminated by the $t$-channel kinematics.
