This study has been conducted to investigate numerically the characteristics of train-induced unsteady airflow in a subway tunnel. A three-dimensional numerical model using the dynamic layering method for the moving boundary of a train is applied. The validation of the present study has been carried out against the experimental data obtained by Kim and Kim [1] in a model tunnel. After this, for the geometries of the tunnel and subway train which are very similar to those of the Seoul subway, a three-dimensional unsteady tunnel flow is simulated. The predicted distributions of pressure and air velocity in the tunnel as well as the time series of mass flow rate at natural ventilation ducts reveal that the maximum exhaust mass flow rate of air through the duct occurs just before the frontal face of a train reaches the ventilation duct, while the suction mass flow rate through the duct reaches the maximum value just after the rear face of a train passes the ventilation duct. The results of this study can be utilized as basic data for optimizi ng the design of tunnel ventilation systems.