We have applied molecular dynamics simulation for various fluid systems to investigate microscopic mechanisms of phase change. The paper reviews the works recently done in our group. (1) Evaporation-condensation dynamics of pure fluids under equilibrium condition : Besides the difference of condensation coefficient among species, its strong temperature dependence is found, which can be explained from the viewpoint of the structure of liquid surface and the molecular exchange. (2) Evaporation-condensation dynamics of pure fluids under non-equilibrium condition : The condensation behavior seems strongly dependent on the temperature and the density of vapor. This kind of molecular simulation will be useful for longstanding questions such as how fast hot vapor condenses on cool liquid surface, or what the evaporation coefficient means. (3) Gas absorption dynamics : CO2 gas absorption mechanism on water surface is similarly analyzed, from the view point of adsorption-desorption dynamics.