Abstract: How to control the dispersion condition is a bottleneck during analysis of the distribution of superfine aluminum hydrate particle size by laser diffraction method. The reasonable dispersion condition was found by Mastersizer 2000 (laser particle size analyzer) as a chief instrument, and through the investigation including dispersion time controlled by extra high power ultrasonic apparatus, dispersing agent and sample addition amount. Experiments proved that the measuring parallelism and accuracy were good under the condition which was founded.