  We analyze the decay $B\to \phi K$ within the framework of QCD-improved
factorization. We found that although the twist-3 kaon distribution amplitude
dominates the spectator interactions, it will suppress the decay rates
slightly. The weak annihilation diagrams induced by $(S-P)(S+P)$ penguin
operators, which are formally power-suppressed by order $(\Lambda/m_b)^2$, are
chirally and logarithmically enhanced. Therefore, these annihilation
contributions are not subject to helicity suppression and can be sizable. The
predicted branching ratio of $B^-\to\phi K^-$ is $(3.8\pm0.6)\times 10^{-6}$ in
the absence of annihilation contributions and it becomes
$(4.3^{+3.0}_{-1.4})\times 10^{-6}$ when annihilation effects are taken into
account. The prediction is consistent with CLEO and BaBar data but smaller than
the BELLE result.
