In-line and cross-flow vortex-induced vibration(VIV) of a circular cylinder is simulated using CFX software, including mass ratio of 7 and 3.24. The lock-in, beat and phase switch phenomena were caught and effects of mass ratio on VIV were discussed. The results show that fluid-structure interaction(FSI) should be considered in the analysis of VIV of a cylinder. The range of lock-in, the maximum transverse amplitude and the velocity to achieve lock-in in the mass ratio of 3.24 are larger than that in the mass ratio of 7.The frequency of in-line VIV is always twice of that of cross-flow VIV when the mass ratio is 7. However, when the mass ratio is 3.24, the frequency of in-line VIV is twice of that of cross-flow VIV in the lower velocity reduction (Ur≤4). And there are two values in the higher velocity reduction, one is twice of frequency of cross-flow VIV, the other is close to that of cross-flow VIV.