  Using the heavy quark effective theory, chiral structure of the $b\rightarrow
c$ charged current can be determined by the semileptonic $\Lambda_b$ decay with
satisfactory theoretical accuracy. We define an asymmetry which is sensitive to
the chirality of the $b\rightarrow c$ charged current. We show that this
asymmetry has no theoretical uncertainty in the heavy quark limit. It is also
shown that $1/m_c$ correction to this asymmetry is small and disappears at the
kinematical point of zero momentum transfer.
