  We propose in this paper a quintom model of dark energy with a single scalar
field $\phi$ given by the lagrangian ${\cal
L}=-V(\phi)\sqrt{1-\alpha^\prime\nabla_{\mu}\phi\nabla^{\mu}\phi +\beta^\prime
\phi\Box\phi}$. In the limit of $\beta^\prime\to$0 our model reduces to the
effective low energy lagrangian of tachyon considered in the literature. We
study the cosmological evolution of this model, and show explicitly the
behaviors of the equation of state crossing the cosmological constant boundary.
