The Gaussian Diffusion Sphere Model (GDSM) was developed and improved to reflect the effects of gravitational settling as well as Brownian diffusion of aerosol particles on deposition velocity onto a face-up flat surface in a laminar parallel flow. The model improvement also includes the applicability of the GDSM to a flat surface of any shape with finite dimensions. When deposition velocity for a face-up circular flat plate of 45 cm diameter, representing e.g. a semiconductor wafer in a laminar parallel flow, was calculated by the GDSM and compared with that by the theory of Liu and Ahn (1987). Particle deposition on semiconductor wafers. Aerosol Science and Technology, 6, 215-224, the agreement was good for the tested particle sizes ranging 0.003-1 mu m and free stream velocities ranging 5-500 cm/s. Based on this result, deposition velocities onto the face-up square flat plates with different orientations in a laminar parallel flow, simulating e.g. photomasks, were predicted. (C) 2010 Elsevier Ltd. All rights reserved.