Systems that clean by high-speed nanodroplet injection have recently been developed; however, elucidating the cleaning mechanism underlying nanodroplet impact is urgently required. In this study, numerical simulations of the high-speed impact phenomenon of two liquid films were performed using the Enskog–Vlasov direct simulation Monte Carlo method. We determine the minimum liquid film thickness for which the water hammer theory is applicable to the maximum pressure generated during liquid film impact. The simulation results show that the water hammer pressure was generated when the liquid film thickness exceeded the compression wavefront thickness generated by the liquid film impact, which is more than ten times the molecular diameter.