  In this paper we consider a quintom model of dark energy with non-minimal
coupling between scalar field and modified gravity which is known $f(R)$
gravity. The Lagrangian for scalar field has been inspired by tachyonic
Lagrangian in string theory. Then we obtain the equation of state (EoS), and
the condition required for the model parameters when $\omega$ crosses over -1.
This model shows that for having $\omega$ across over -1, one doesn't need to
add some higher derivative operator in the tachyonic part of action (the way
that usually used to obtain crossing of the phantom divide line for EoS
parameter).
