  In this paper, we present the exact calculations for the vertex
$\bar{s}\gamma b$ and $\bar{s} Z b$ in the unitary gauge. We found that (a) the
divergent- and $\mu$-dependent terms are left in the effective vertex function
$\Gamma^\gamma_\mu(p,k)$ for $b \to s \gamma$ transition even after we sum up
the contributions from four related Feynman diagrams; (b) for an on-shell
photon, such terms do not contribute et al; (c) for off-shell photon, these
terms will be canceled when the contributions from both vertex $\bar{s}\gamma
b$ and $\bar{s} Z b$ are taken into account simultaneously, and therefore the
finite and gauge independent function $Z_0(x_t)=C_0(x_t)+ D_0(x_t)/4$, which
governs the semi-leptonic decay $b \to s l^- l^+$, is derived in the unitary
gauge.
