Abstract: In order to investigate the structure of blade tip vortices, Panicle Image Velocimetry (PIV) measurements were conducted on tip vortices trailing from the rotor with slotted blade to quantify the swirl velocity, core radius and vorticity profile at a fixed wake angle of 145 degree. The influence of the PIV image analysis parameters, e.g. sampling window size, on the vortex structure parameters has been investigated. To mitigate the effects of vortex wander, a scheme to align the velocity fields based on the locations where the velocity minimum appears prior to averaging was developed. Compared to simple average, the conditional average method obtains more accurate parameters of the vortex.