External stresses cause certain proteins to lose their regular structure and aggregate. In order to clarify this abnormal aggregation process, a structural evolution of human recombinant alphaB-crystallin under UV irradiation was observed with in situ small-angle neutron scattering. The abnormal aggregation process was identified to fall in three time zones: incubation, aggregation, and saturation. During the incubation time, the size of aggregates was almost unchanged but a deformation in the local structure was developing. After the incubation time, abnormal aggregation proceed. When the volume of the aggregates reached around twice the size as that of the initial aggregates, the aggregation rate slowed down, which marked the onset of saturation.