The forces acting on and the flow patterns around a rigid bluff body submerged in an oscilatory flow were studied. The force coefficients on a circular cylinder were measured and compared with those from other studies using a U-tube water tank to verify the experimental setup. Then, measurements of both in-line and transverse forces of a square cylinder at an angle of attack of 0° were made in a relatively wide range of Keulegan-Capenter(KC) numbers from 1 to 90. For both circular and square cylinders, the force signals were analyzed spectrally to obtain major frequencies. Flow visualization was also employed to confirm the relationship between flow patterns and force coefficients. It is evident that several classes of flow patterns appear within the measured KC range, and that variations of the in-line and transverse forces are related closely to vortex motions.