In this paper the scalar-tensor theory of gravity is assumed to describe the evolution of the universe and the gravitational scalar $phi$ is ascribed to play the role of inflaton. The theory is characterized by the specified coupling function $omega(phi)$ and the cosmological function $lambda(phi)$. The function $lambda(phi)$ is nearly constant for $0<phi<0.1$ and $lambda(1)=0$. The functions $lambda(phi)$ and $omega(phi)$ provide a double-well potential for the motion of $phi(t)$. Inflation commences and ends naturally by the dynamics of the scalar field. The energy density of matter increases steadily during inflation. When the constant $Gamma$ in the action is determined by the present matter density, the temperature at the end of inflation is of the order of $10^{14} GeV$ in no need of reheating. Furthermore, the gravitational scalar is just the cold dark matter that men seek for.
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