CFD analysis is performed for a labyrinth oil mist separator in a three-cylinder diesel engine with CFD software package AVL-FIRE. The velocity and pressure distribution of the labyrinth is calculated. The discrete model is used to simulate the injection of the oil mist particles. Oil mist particles are assumed to be trapped once they impact the wall, then the separation efficiency of different size of oil mist is obtained. The pressure drop and oil mist separation efficiency of several different labyrinth oil mist separator are compared. The results show that though the simulation results are higher, the trend is consistent with that of test results, which proves the feasibility of using CFD analysis on the simulation of oil mist separator.