  We propose a new dihedral angle observable to measure the CP property of the
interaction of top quark and Higgs boson in the $t\bar{t}H$ production at the
14~TeV LHC. We consider two decay modes of the Higgs boson, $H\to b\bar{b}$ and
$H\to \gamma\gamma$ and show that the dihedral angle distribution is able to
distinguish the CP-even and the CP-odd hypothesis at 95\% confidence level with
an integrated luminosity of $\sim 180~{\rm fb}^{-1}$.
