An experimental study was made about an elastically mounted cylinder exposed to a periodic fluid flow field. The corresponding analysis placed emphasis on the prediction of fluid lift forces and cylinder responses normal to the deterministic, sinusoidal oscillating flow. A cylinder model was driven sinusoidally in still fluid in order to give a relative sinusoidal motion between the fluid and cylinder. This experimental arrangement was selected due to both a simple implementation of experimental apparatus and a strict control of the input variables such as cylinder driving amplitude and frequency. The corresponding fluid lift forces and the cylinder response amplitude were measured for two different size cylinder models (D = 1 inch, and D = 2 inch) for various cylinder driving frequencies and amplitudes;For a stationary cylinder in an oscillating fluid flow, it is well-known that the fluid lift force is a function of Reynolds number;(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI);and Keulegan-Carpenter number;(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI);where U(,m) is the maximum fluid oscillating velocity, D is the cylinder diameter, (nu) is the fluid kinematic viscosity and A is the fluid oscillating amplitude;On the other hand, for an elastically mounted cylinder, the vortices generated behind cylinder model can excite the cylinder model and, in turn, the vibrating cylinder can affect the vortex formation and the fluid lift forces. It is believed that the fluid force estimation based on stationary cylinder data must be in error compared to the actual situation. In this experiment, it was found that the fluid lift forces and the cylinder response amplitudes for an elastically mounted cylinder can be explained and predicted using frequency ratios such as f(,n)/f(,d), f(,r)/f(,d), and f(,v)/f(,d), where f(,n) is the cylinder natural frequency, f(,r) is the cylinder response frequency in the y-direction, f(,v) is the vortex shedding frequency, and f(,d) is the cylinder driving (fluid oscillating) frequency, rather than the Reynolds and the Keulegan-Carpenter numbers. When f(,r)/f(,d) = f(,v)/f(,d) = f(,n)/f(,d) = integer, there occurs a sharp increase of the cylinder response amplitudes and the fluid lift forces. The measured fluid lift forces at this integer relationship region were found to be far greater than those corresponding to a stationary cylinder exposed to an oscillating flow field. The locked-on phenomenon and the natural frequency variations due to the cylinder driving conditions were also closely examined.