  $Om$ diagnostic can differentiate between different models of dark energy
without the accurate current value of matter density. We apply this geometric
diagnostic to dilaton dark energy(DDE) model and differentiate DDE model from
LCDM. We also investigate the influence of coupled parameter $\alpha$ on the
evolutive behavior of $Om$ with respect to redshift $z$. According to the
numerical result of $Om$, we get the current value of equation of state
$\omega_{\sigma0}$=-0.952 which fits the WMAP5+BAO+SN very well.
