The study is conducted in order to understand a mechanism of interphase mass transfer between liquid and vapor. The molecular dynamics (MD) simulation is used to examine details of condensation and evaporation from a viewpoint of molecular kinetics. First, molecular boundary conditions for condensing, reflecting and evaporating molecules are presented for argon molecule as a function of the surface-normal component of translation energy. The velocity distributions can be expressed by the modified Maxwellian with making use of the condensation coefficient. The condensation coefficient of water is also examined for two kinds of intermolecular potential, the Carravetta-Clementi (C-C) model and the Extended Simple Point Charge (SPC/E) model, in order to consider the effect of the surface structure of liquid on the condensation coefficient. The results indicate that the condensation coefficient of water is close to unity for both model and its dependency on the translation energy is small compared with argon. Finally, the condensation coefficient is studied based on the transition state theory. An evaluation of the transition state is considered by applying the results of MD simulations for argon and water.