A bluff structure placed in a flowing fluid, may be subjected to vortex-induced vibrations (VIV). For a flat plate with only rotational degree of freedom, the VIV is rotational oscillation. Based on the experimental investigation, vortex-induced oscillation of the plate is studied. The Strouhal number is measured from the stationary plate in a low speed steady wind tunnel. A set of vibration tests are conducted to investigate the relationships between shedding frequency and vibration frequency. “lock-in” phenomena is observed with and without large amplitude. An empiricalanalytical model via introducing a nonlinear van der Pol oscillator is developed. This thesis investigates the “lock-in” phenomena of a flat plate in pitch oscillation. Results from wind tunnel experiments on a flat plate indicate the “lock-in” is frequency “lock-in”, resonance which appears large response amplitude occurs in the “lock-in” regime and may be influenced by “lock-in” phenomena.