Abstract: Vortex-induced vibrations of three tandem cylinders at the spacing ratio of 1.2 and the Reynolds number of 100 shows that when the reduced velocity is larger than a critical value, the galloping is observed where the amplitudes of three cylinders increase with the reduced velocity. Three factors, including shift of equilibrium position, low vibration frequency and timing between vortex shedding and motion of cylinders, determine the appearance of galloping. Further investigations show that the galloping occurs at a range with lower mass ratio no more than 2.0 and Reynolds number no more than 100. When the mass ratio is large, the equilibrium position is unchanged and the displacements are irregular, which leads to the variation of timing between vortex shedding and motion of cylinders. When Re is large, the shift of the most upstream cylinder is zero and no more accommodate cylinders’ vibration, which makes the vibration irregular and change of timing between vortex shedding and motion of cylinders.