  Recent observations confirm that our universe is flat and consists of a dark
energy component with negative pressure. This dark energy is responsible for
the recent cosmic acceleration as well as determines the feature of future
evolution of the universe. In this paper, we discuss the dark energy of the
universe in the framework of scalar-tensor cosmology. In the very early
universe, the gravitational scalar field $\phi$ plays the roll of the inflaton
field and drives the universe to expand exponentially. In this period the field
$\phi$ acts as a cosmological constant and dominates the energy budget, the
equation of state (EoS) is $w=-1$. The universe exits from inflation gracefully
and with no reheating. Afterwards, the field $\phi$ appears as a cold dark
matter and continues to dominate the energy budget, the universe expands
according to 2/3 power law, the EoS is $w=0$. Eventually, by the epoch of
$z\sim O(1)$, the field $\phi$ contributes a significant component of dark
energy with negative pressure and accellerates the late universe. In the future
the universe will expand acceleratedly according to $a(t)\sim t^{1.31}$.
