Flow properties of steam and their effect on separation efficiency of droplets in mist separators are discussed based on computer simulation. The scales of the separators reflect units installed in a geothermal power plant. The diameters of the inner and outer cylinders are 1 and 3 m, respectively, and the diameter of the inlet pipe is 0.8 m. The distance between the inlet pipe and the outlet mouth, H, is varied between 3.4 and 13.4 m. The average velocity of steam in the inlet pipe is 22 m/s. The steam including droplets is tangentially introduced via the inlet pipe. The steam then forms rotating vortices in the annular section of the separator, moves upwards and finally enters the downward outlet pipe. The droplets are transported toward the inner wall of the outer cylinder by centrifugal force due to the rotating vortices. In the separator of H = 3.4 m, the minimum droplet size which is trapped is 20-30 mum. In the separator of H = 13.4 m, the steam forms stable rotating vortices, the diameter of which decreases with increasing vertical position. This flow towards the inside of the annular space partially cancels the centrifugal force. Thus the critical droplet size does not significantly change. From the practical point of view, the optimum separation height, H, is estimated to be 7-8 m.