Molecular dynamics simulations have been conducted for a system of argon molecules to study the effects of translational motion of monatomic molecules on condensation and evaporation coefficients. The results show that both coefficients depend on a surface-normal component of the translational energy. The vapor molecules with smaller energy can be easily reflected by the surface molecules, and the surface molecules with smaller energy can not evaporate. Also, velocity distributions of evaporated and reflected molecules are presented in the form of modified Maxwellian utilizing the condensation/evaporation coefficient expressed as a function of the normal component of translational energy.