We use molecular dynamics (MD) simulations to study the transient evaporation and condensation of a pure fluid Ar in a nanochannel. In the MD model, the evaporation and condensation of fluid Ar is initiated by a sudden increase of the temperature or periodically varying the temperature in the solid substrate on one side of the nanochannel. In both cases, we find the transient evaporation and condensation rates obtained directly from MD simulations are in good agreement with the predictions from the Schrage relationships. Furthermore, our analyses show that the kinetics of the transient heat and mass transfer between the evaporating and the condensing surfaces in the nanochannel are mainly controlled by heat and mass diffusion in the vapor rather than by convection. The simulation results indicate that the Schrage relationships are capable of accurately describing the transient evaporation/condensation processes and their rates even under a high-frequency oscillatory driving force condition.