Abstract: The research has been made on the determination conditions of particle size of nano-sized BaSO4 by Zetasizer Nano S90 laser grainsize analyzer.The influence of ultrasonic time on determination result was firstly studied.On the basis of that， the effects of dispersants，concentration of dispersant，dispersion medium，and content of nano-sized BaSO4 solution on particle size determination were investigated，thus the optimum determination conditions were obtained as follows：the ultrasonic dispersing time was 15 min，sodium hexametaphosphate with mass concentration of 1 g/L as the dispersant，dispersion medium was distilled water，and the mass concentration of nano-sized BaSO4 solution was 0.35 g/L.The measured results of laser grainsize analyzer were basically in correspondence with the characterization results of scanning electron microscopy.

Key words: laser grainsize analyzer, barium sulfate, particle size analysis, dispersant