Abstract: Flow-induced vibration of two side-by-side circular cylinders at Re= 100 is numerically investigated. The cylinders are constrained to oscillate in the cross-flow direction with a center-to-center spacing ratio s/D = 2.5 and 5.0. The mass ratio of the cylinders is 2.0 and the reduced velocity is U_r= 2.0 ～ 10.0. The results show that, for the case with s/D = 2.5, asymmetric vibrations of the two side-by-side cylinders is observed in 4.0 < U_r< 4.8, while the symmetry hysteresis is observed in 4.4 < U_r< 4.8. However, for the case with s/D = 5.0, both two phenomena disappear and the vibration responses of two side-by-side circular cylinders are close to those of an isolated circular cylinder. Moreover, it was found that the lift and drag coefficients of the two cylinders also showed asymmetric features in the asymmetric vibrations region. When the two cylinders oscillate with different amplitudes, the gap flow stably biases to one of the cylinders and thus leads to a wide-narrow near-wake pattern. The cylinder with the narrower near-wake has larger vibration amplitude, drag and lift forces than the other one with the wider near-wake. A detailed explanation on the mechanisms of the asymmetric vibration and symmetry hysteresis is presented from the point of view of near-wake patterns before and after the asymmetric vibration occurs.