The objective of this work was to propose a new and simple method to understand the hydrodynamic phenomena that occur within two types of hydrocyclones, cylindrical and conical. Using a PCO 1200 hs digital high-speed camera, this method involved the following of a little red oil droplet that had been injected by a small syringe into the hydrocyclone at different zones. The trajectory line of the injected droplets and velocity of the water were investigated, in terms of the tangential and axial velocities of the water and the average velocity gradient in the hydrocyclone. In both hydrocyclones, if the oil droplets were injected at the center of the hydrocyclone, they went down to the bottom part of the hydrocyclone. In contrast, if the droplets were injected close to the wall of the hydrocyclone, they rose rapidly to the upper part of the hydrocyclone. Moreover, the tangential velocity of water in both hydrocyclones presented average velocity gradient of the water at the center of the hydrocyclone which was significantly lower than the average velocity gradient of the water close to the wall of hydrocyclone. Therefore, it could be concluded that the external vortex close to the wall of the hydrocyclone was stronger than the internal vortex at the center of the hydrocyclone. These obtained results are in agreement with those observations in the literatures. In conclusion, we propose this simple experimental method as an effective means of understanding the hydrodynamic phenomena that occurs within hydrocyclones and so improving the hydrocyclone design for further applications. (C) 2008 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers.