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 \frac { 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 } . PACS number ( s ) : 98.80.Cq , 04.50.+h