Abstract: Experiments on a square-sertion cylinder fixed and forced to oscillate transversely in uniform stream were conducted in a water tank. The Reynolds number of the experiments is in the range 3·103 to 104, the amplitude to side length ratio A/D is up to 0.7 and the range of reduced velocity 4.5≤Vt≤12.This study aims at investigating the lock-in phenomenon, fluctuating lift and the phase shift between fluctuating lift and displacement of the oscillating cylinder. The problems on the aeroelastic instability rela...