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Evolution of the unsteady three-dimensional tip vortex in the wake field of a rectangular NACA 0012 hydrofoil in pitching motion is investigated. Measurements were made in CWC using PIV. A hydrofoil has an aspect ratio of 5 with chord length of 10cm. Pitching angle and mean angle of attack were set to 5° and 10°, respectively. Frequency of oscillation was varied from 0.1Hz to 1Hz in order to study the effect of unsteadiness imposed by various frequencies, which correspond to the reduced frequency of K=0.1, 0.21, 0.52 and 1.05. Reynolds number based on chord length and free-stream velocity was 3.0×104. Phase-averaging technique was employed. Unsteadiness and variation of the size and characteristics of tip vortex at different reduced frequency were discussed.