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 .

PACS : 04.20.-q ; 98.80.Cq .