Purpose of the present study is to understand the mechanism of interphase mass transfer between liquid and vapor under non-equilibrium conditions. The molecular dynamics (MD) simulation is used to examine details of condensation and evaporation from a viewpoint of microscopic description. The simulations are made for a monatomic-molecule system consisting of 6912 argon molecules expressed by the Lennard-Jones intermolecular potential. Velocity distributions of the leaving molecules from the liquid surface are obtained and the effects of non-equilibrium conditions are discussed. We found that the reflection on the condensing surface show differences from equilibrium results since the incident molecules have mean velocity due to the net mass transfer rate. On the other hand, the condensation coefficient and the velocity distribution functions for the evaporated molecules show no dependence on the non-equilibrium conditions. Furthermore, the energy accommodation coefficients of the reflecting molecules are examined and compared with each other.