Direct observation of the powder structure in aqueous alumina slurry was investigated using optical microscopy in the transmission mode. In this technique, the slurry was sandwiched by two glass plates kept a 100 mu m apart and the structure of powder agglomerates in the slurry was projected under optical transmission mode. It was revealed that the technique is powerful for examining large-scale heterogeneity in aqueous ceramic slurry of which the solid concentration is up to 10 vol.%.