  We investigate $\ovar{B^0} \to D_s^- \pi^+$ decay in perturbative QCD
approach which has recently been applied to $B$ meson decays. $\ovar{B^0} \to
D_s^- \pi^+$ decay (and its charge conjugated mode) can be one of the hopeful
modes to determine $|V_{ub}|$ since it occurs through $b \to u$ transition
only. We estimate both factorizable and non-factorizable contribution, and show
that the non-factorizable contribution is much less than the factorizable one.
Our calculation gives ${BR}(\ovar{B^0} \to D_s^- \pi^+) = (50 \sim 70) \times
f_{Ds}^2|{V_{ub}}{V_{cs}}|^2$.
