  We investigate the issues of future oscillations around the phantom divide
for $f(R)$ gravity. For this purpose, we introduce two types of energy density
and pressure arisen from the $f(R)$-higher order curvature terms. One has the
conventional energy density and pressure even in the beginning of the Jordan
frame, whose continuity equation provides the native equation of state $w_{\rm
DE}$. On the other hand, the other has the different forms of energy density
and pressure which do not obviously satisfy the continuity equation. This needs
to introduce the effective equation of state $w_{\rm eff}$ to describe the
$f(R)$-fluid, in addition to the native equation of state $\tilde{w}_{\rm DE}$.
We confirm that future oscillations around the phantom divide occur in $f(R)$
gravities by introducing two types of equations of state. Finally, we point out
that the singularity appears ar $x=x_c$ because the stability condition of
$f(R)$ gravity violates.
