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 $tanbeta$. 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 $tanbeta$. We preform a detailed analysis of measuring top-quark polarization in the production channels $gbto tH^-$ and $gbar{b}to bar{t}H^+$. 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 $tanbeta$, especially for the intermediate $tanbeta$ region. On the contrary, the top-quark produced in association with the charged Higgs boson cannot be used to measure $tanbeta$ because its polarization is highly contaminated by the $t$-channel kinematics.
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