  We compute branching ratios of $B\to K\pi$ decays in the framework of
perturbative QCD factorization theorem. Decay amplitudes are classified into
the topologies of tree, penguin and annihilation, all of which contain both
factorizable and nonfactorizable contributions. These contributions are
expressed as the convolutions of hard $b$ quark decay amplitudes with universal
meson wave functions. It is shown that (1) matrix elements of penguin operators
are dynamically enhanced compared to those employed in the factorization
assumption; (2) annihilation diagrams are not negligible, contrary to common
belief; (3) annihilation diagrams contribute large strong phases; (4) the
uncertainty of current data of the ratio $R={\cal B}(B_d^0\to
K^\pm\pi^\mp)/{\cal B}(B^\pm\to K^0\pi^\pm)$ and of CP asymmetries is too large
to give a constraint of $\phi_3$. Assuming $\phi_3=90^o$ which is extracted
from the best fit to the data of $R$, predictions for the branching ratios of
the four $B\to K\pi$ modes are consistent with data.
