Abstract: Chemical kinetics theory is used to calculate the effect of electrons in plasma on the generating rate of atomic hydrogen, methyl and acetylene molecules in the depositing atmosphere during microwave plasma-assisted chemical vapor deposition of diamond thin film. It is found that the accelerating effect of plasma on gas-phase reactions is mainly realized through its super-equilibrious electrons. The electrons can dissociate hydrogen molecule into hydrogen atoms efficiently and provide another important reaction rotes for the dissociation of methane molecule into methyl, and therefore, accelerate all gas-phase reactions in the depositing atmosphere.