Deposition velocity onto a 450-mm-flat-plate in parallel airflow was numerically investigated by considering the influence of simultaneous electrophoresis and thermophoresis on particles in charge equilibrium. Both the face-up and face-down orientations of the flat plate were considered. When the electric field strength was 0 V/cm, the deposition velocity of particles in charge equilibrium was greatly influenced by thermophoresis, in the size range between 0.005 mu m and 3 mu m. With increasing electric field strength, the deposition velocity of particles in charge equilibrium under the existence of thermophoresis increased for both the face-up and face-down surfaces and the size range of particles affected by thermophoresis became narrower. The electric field of 1000 V/cm appeared to make the deposition velocity of particles in charge equilibrium almost unaffected by thermophoresis. For the face-down surface, the deposition velocity of micrometer-sized particles dropped sharply even with the attractive thermophoresis and electrophoresis, owing to gravitational settling effect. With a strong electric field applied, the deposition velocity of particles smaller than approximately 3 mu m onto both the face-up and face-down surfaces was expected to be high, even thermophoresis was repulsive. (C) 2013 Elsevier Ltd. All rights reserved.