  The heavy quark effective field theory (HQEFT) containing both effective
`quark fields' and `antiquark fields' is investigated in detail. By integrating
out (but not neglecting) the effective antiquark fields, we present a new
formulation of effective theory which differs from the usual heavy quark
effective theory (HQET) and exhibits valuable features because of the inclusion
of the contributions from the antiquark fields. Matrix elements of vector and
axial vector heavy quark currents between pseudoscalar and vector mesons
containing a heavy quark (b or c) are then evaluated systematically up to the
order of $1/m^2_Q$ and parameterized by a set of universal form factors. With a
consistent normalization condition between the effective heavy hadron states,
the form factors at zero recoil are related to the ground state meson masses,
which enables us to estimate the values of form factors at zero recoil. In
particular, the Luke's theorem comes out automatically in the new formulation
of HQEFT without the need of imposing the equation of motion $iv\cdot D
Q^{+}_{v} =0$. Consequently, the differential decay rates of both $B\to
D^{\ast}l\nu$ and $B\to D l\nu$ do not receive $1/m_{Q}$ order corrections at
zero recoil, which is not the case in the usual HQET. Thus we quote that the
Cabibbo-Kobayashi-Maskawa matrix element $| V_{cb}|$ can nicely be extracted
from either of these two exclusive semileptonic decays at the order of
$1/m^2_Q$. Our estimates for $|V_{cb}|$ are presented.
