In situ solidification was utilized to fix particle dispersion in slurries. Solidified alumina slurries were thinned and observed with an optical microscope. By increasing the amount of dispersant, significant changes in particle dispersion were successfully observed even in the range where the viscosity of slurry was substantially constant. Particles were stereoscopically networked with insufficient dispersant addition, nearly monodispersed at an appropriate amount of dispersant and reagglomerated by adding excess amount of dispersant. Transparent analysis using optical microscopy was quite effective for visualizing the stereoscopic structure of the particles. We also propose an easy method of microscopic observation without special sample preparation.