The effect of pancreatic hydrolysis on the aggregate size and molecular weight distribution (MWD) of bovine casein was investigated by size exclusion chromatography (SEC), coupled with multiangle laser light scattering (MALLS) and dynamic light scattering (DLS). The concentrations of both casein aggregates and monomers decreased as the reaction proceeded and almost disappeared after a 10min hydrolysis. The ratio of aggregates to monomers was maintained at 1:(2-2.5) during the initial 10min hydrolysis, which shows that the caseins in the solution are in a dynamic equilibrium system, between aggregates and monomers. Upon limited hydrolysis, casein aggregates displayed a continuous growth in their sizes and molecular weights, together with a decrease in the intermolecular repulsion. This result was further supported by on-line DLS measurements, which showed the increase in the hydrodynamic radii and the decrease in light scattering intensities of casein hydrolysates. Moreover, the release kinetics of peptide fractions with different molecular weights were also followed. It was suggested that the increase in the hydrophobic attraction and the reduction in the protein repulsion might promote casein aggregation, during the limited pancreatic hydrolysis.