Recent observations confirm that our universe is flat and consists of a dark energy component $Omega_{DE}simeq 0.7$. This dark energy is responsible for the cosmic acceleration as well as determines the feature of future evolution of the universe. In this paper, we discuss the dark energy of universe in the framework of scalar-tensor cosmology. It is shown that the dark energy is the main part of the energy density of the gravitational scalar field and the future universe will expand as $a(t)sim t^{1.3}$.