  We calculate the tree and penguin amplitudes in the $B^0\to\pi^+\pi^-$ decay
channel employing the perturbative QCD factorization approach. Using the
amplitudes as input with the theoretical uncertainties sufficiently considered,
we constrain the UT angle $\gamma$ to $53^\circ\leq\gamma\leq70^\circ$, from
the measurements of the CP violation parameters $C_{\pi^+\pi^-}$ and
$S_{\pi^+\pi^-}$ in $B^0\to\pi^+\pi^-$. The U-spin breaking effect between
$B^0\to\pi^+\pi^-$ and $B_s^0\to K^+K^-$ is estimated to be around 30\%.
