First, the present study deals with the visualization of flow fields around and behind a 2-D & 3-D NACA0012 airfoil oscillating with various frequencies. Since the unsteadiness of flow is then known to be depending on the reduced frequency K, the airfoil was oscillated by using a servo-motor which could be computer-controlled at wide range of frequencies and amplitudes. It was tentatively found that the gloval flow field was greatly altered by the reduced frequency and the mean angle of attack by the frequency. Results contained in this paper could provide the valuable information on the study for fluid-structure interaction. And then, sn experimental study was carried out to examine quantitatively the vortical flow fields around and behind an oscillating 2-dimensional airfoil by phase. A NACA0012 airfoil with an aspect ratio of 4.0 was mounted on the middle section of the CWC (Circulating Water Channel), and the airfoil was sinusoidally pitched about the quarter chord. The mean angle of attack was set at various angles and the amplitude of oscillation was set at 2.5. The flow visualization and measurements were made for depending on the reduced frequency K, and Reynolds number Rn. This paper introduces the results by an experiments and measures, and that compares case the ignore phase and case that is not so. Last, 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. An airfoil 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. (i.e. the angle of attack varies from 5° to 15°. 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 1.06. 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.