This paper describes an easier and more effective method for producing a colloidal solution in which flocculated nanoparticles become re-dispersed by using a back pressure valve. Initially, a solution of nanoparticles (CdSe/ZnS-core/shell quantum dots) is left to flocculate by storing water-soluble nanoparticles (modal diameter 13.5 nm) for 2 d at 40 degrees C. A large shear stress is applied to the solution in a back pressure valve for re-dispersing the flocculated particles. The clearance of the flow path in the valve decreases with increasing primary pressure. The re-dispersibility is evaluated through the measurement of their size distribution and zeta potential using dynamic light scattering (Zetasizer). The results show that the method re-dispersed the flocculated particles and reduces their modal diameter from over 6000 nm to 21.0 nm. Additionally, increasing the pressure decreases the particle diameter after the re-dispersion. The re-dispersed particles have a larger effective surface area than those of flocculated particles and, therefore, a larger zeta potential. In addition, we identify the mechanism of re-dispersion in this valve from the ratio of the applied shear stress or the colliding force against the channel wall compared to the van der Waals interaction energy. From the results, shear stress dominates the re-dispersion of flocculated particles. Moreover, we can predict the particle diameter after the re-dispersion from the dispersion numbers Di(1) (the ratio of the shear stress to the van der Waals interactive energy) and Di(2) (the ratio of the collision force to the interactive energy). Finally, the re-dispersed particles are confirmed to provide the equivalent medical activity to that of the samples immediately after synthesis.