In order to confirm reliable particle size measurement technique and to prepare standard reference particles for calibrating particle size measurement devices, experimental and theoretical studies have been conducted about particle size measurement of 0.1-1 mu m silica particles. The microscopic method with sample size greater than 90,000 particles was conducted for the size measurement. Theoretical equation of uncertainty region over all particle diameter range is newly proposed and compared with computer simulation. Previous paper [H. Masuda, K. Iinoya, Theoretical study of the scatter of experimental data due to particle size distribution, J. Chem. Eng. 4(l) (1971) 60-67] reported the uncertainty region only for mass median diameter, but this paper presents the uncertainty region for all particle size range. The uncertainty region increases with the increase in particle diameter and also increases as the sample size decreases. Theoretical uncertainty region agreed with the results of computer simulation. (C) 2008 The Society of Powder Technology Japan. Published by Elsevier BV and The Society of Powder Technology Japan. All rights reserved.