Flow patterns, dust concentration profile, and particle motion in a mountain tunnel under construction were calculated numerically for a full-scale tunnel to evaluate the effectiveness of the planned ventilation system. The influence of ventilation air flow rate, the configuration of air tubes, and an obstacle near the working face were investigated. The trajectories of different size particles were calculated at different wall conditions for deposition. A vortex flow was found to form between the air inlet and the working face for all ventilation types examined. The average dust concentration at a height of 1.5 m, corresponding to the average breathing height of a worker, did not consistently decrease with an increased air flow rate in an injection-suction type system. An optimal air flow rate for minimizing the dust concentration may exist. A vortex flow developed around an obstacle near the working face, leading to an increase in dust concentration between the obstacle and the working face. The concentration of dust near the working face was extremely high and was too spatially variable to be accurately described by the average dust concentration in the area between the working face and the air inlet. The fraction of particles removed through the air outlet was dependent on the ventilation pattern, and also decreased with increasing particle size due to immediate deposition of coarse particles on the tunnel floor.