  Motivated by recent measurements, we investigate $B\to \pi \pi, K \pi$ decay
modes in the framework of QCD improved factorization, which was recently
proposed by Beneke $et al$. We find that all the measured branching ratios are
well accommodated in the reasonable parameter space except for $B\to K^0 \pi^0
$. We also discuss in detail the strong penguin contributions and the ${\cal
O}(\alpha_s)$ corrections to the chirally enhanced terms. We find that the weak
phase $\gamma$ lies in the region $120^{\circ}<\gamma<240^{\circ}$, which is
mainly constrained by $B\to \pi^- \pi^+ $.
