Oiling out during the crystallization process often generates agglomeration. This research is focused on the control of agglomeration in the antisolvent crystallization of the propylparaben-ethanol-water system in the presence of oiling out, that is, the liquid-liquid phase separation phenomenon. Crystallization trajectories were designed to start from a homogeneous solution with different initial concentrations of propylparaben in ethanol, crossing the liquid-liquid phase separation region in the ternary phase diagram by adding the antisolvent of water. A combination of process analytical technology tools, such as focused beam reflectance measurement, particle visual monitoring, and attenuated total reflectance ultraviolet/visible, enabled the detection of the different stages of the oiling out crystallization process, including the droplet formation of the dispersed phase, prenucleation, nucleation, and crystal growth both in the liquid-liquid phase-separated system and in the homogeneous solution. The crystals tended to form normally at higher initial concentrations of propylparaben while tending to agglomerate at a lower initial concentration. Crystallization experiments were also performed with the addition of sonication, with variation in sonication start time, duration, and power. The results showed that ultrasound could reduce the agglomeration dependent on the initial application time. The mechanism of crystallization and agglomeration in oiling out was discussed.