Abstract: Vortical structures in tip clearance flow are prone to induce the flow cavitation for hydraulic machineries such as ducted propellers and hydroturbines. Unlike the tip vortical flow in conventional vessel propellers, the tip clearance flow is affected by the load distribution on the blade and the gap size between the blade tip and the end wall. To study the characteristics of vortical structures in tip clearance flow, the visualization by vortex cavitation bubbles and the velocity measurement by 2D-3C PIV are carried out for the NACA0024 straight airfoil in a cavitation tunnel. The global distribution of vortical structures in the tip clearance flow and the influence of gap width, Reynolds number, and load coefficient on the vortex strength are obtained by cavitation observation experiment. In addition, the vortex distribution in local flow fields without cavitation and its variation with different influence factors are obtained by velocity measurement. By combining the flow visualization and velocity measurement results, the morphology of vortical structures and their formation mechanism are analyzed. And the effects of the gap width, Reynolds number, and load coefficient on the vortex strength are illuminated preliminarily. Results show that there are two types of steady vortical structures in the tip clearance flow, i.e., the tip separated vortex and tip leakage vortex, which are responsible for inducing the cavitation of tip clearance flow. The angle of attack and gap size of the airfoil will affect the strength of vortices in the tip clearance flow significantly.