By statistically analyzing the condensation process and reconsidering the transition state theory for condensation and evaporation, we first presented a completed theoretical formula of the condensation coefficient. Namely, the unknown parameters contained within the transition state theoretical calculation of the condensation coefficient are determined. The condensation coefficients calculated from this formula agree well with those from molecular dynamics simulations. With this formula, the classical expression of the condensation flux developed from the gas kinetic theory can be used to predict the condensation flux.