Abstract The effect of the vortex flow by primary instability on the precipitation of barium sulfate crystals was examined in Taylor–Couette crystallizers. Experiments involving precipitation of barium sulfate under laminar flow (before the onset of instability) and vortex flow (after the onset of primary instability) at three different axial flow rates revealed that the vortex motion plays a critical role in fine-tuning crystal properties such as internal crystal structure, particle microstructure and morphology. While no clear trends in crystalline size were observed for laminar flow, all flowrates at the onset of instability resulted in smaller crystalline sizes that ranged between 40nm and 50nm. A decrease in the unit cell dimensions of the orthorhombic lattice structure was also observed after the onset of instability. For both laminar and vortex flow, shortening the residence time by increasing axial flow rate led to a substantial decrease in crystal size. It is also observed that the crystal size distribution (CSD) became much narrower after the emergence of vortices. Under vortex flow, an increase in axial flow resulted in even narrower crystal size distributions. Finally, the observed phenomena of smaller crystal size and narrow crystal size distribution under vortex flow are qualitatively explained by the analysis of particle trajetory.