The rate equation for Si etching in a CF4 plasma where a polymer film consisting of the C-C bond does not grow on the substrate was derived based on a simple kinetic model. The etch rate was expressed as a function of four internal process variables: the sheath bias voltage, the ion current density, and the partial pressures of atomic fluorine and the CFx polymer precursors. Etching experiments were made in a transformer coupled plasma etcher over a wide range of process conditions measuring the internal variables in situ. The rate constants of the proposed reaction steps, estimated by fitting the etch rate equation to the experimental results, had reasonable values when compared with the values reported in the literature. The derived etch-rate equation allowed correlation between the reaction probability of F atoms with Si and the fractions of the surface species: Si(c), SiF2(a), and Si(CFx)(2)(a).