As well known, a size distribution affects the packing proper-ties of a fine powder, but no systematic data can be found about the effect of size or size distribution on packing density of fine powder under 50 mum. Fine powders with log-uniform size distribution were prepared by mixing of mono-sized silica or fly ash powder that was classified by Turbo classifier. The tapping property of the powder packed beds was measured using the Powder tester. From our experimental results, void fraction of the tapped powder beds is affected by size distribution in all tapping numbers. For log-uniform size distribution, the value of void fraction decreases with decreasing inclination of the size distribution. In the case of Andreasen (Gaudin-Shuhmann) distribution, the minimum void fraction was obtainable at the Fuller constant of 0.7-0.9. The experimental results were compared with the estimated values by our proposed model for a multi-component particle mixture. (C) 2001 Elsevier Science B.V. All rights reserved.