  Inspired by the unexpectedly large difference between the CP violation of $D$
decays into $K^+K^-$ and $\pi^+\pi^-$, we explore the impact on the extraction
of $\gamma$ via the $B\to DK$ process with the $D$ meson reconstructed in the
$K^+K^-,\pi^+\pi^-$ final state. We show that the extracted results for
$\gamma$ can be shifted by ${\cal O}(A_{CP} / r_B^K)$, where $A_{CP}$ is the
direct CP asymmetry in $D$ decays and $r_B^K$ is the ratio of the decay
amplitudes of $B^-\to \bar D^0K^-$ and $B^-\to D^0K^-$. Using the recent data
on CP asymmetry, we demonstrate the correction to physical observables in $B\to
DK$ can reach 6%, which corresponds to the shift of $\gamma$ by roughly
$5^\circ$. The remanent corrections depend on the strong phase of the $D$
decays, but are less than $0.5^\circ$. With the increasing precision in the
$\gamma$ determination on the LHCb experiment and Super B factories, the
inclusion of CP violation of $D$ decays will therefore soon become important.
