Based on the FEM solution of incompressible viscous Navier-Stokes equations in the frame of Arbitrary Lagrangian-Eulerian (ALE), a two-dimensional numerical model is developed to investigate the vortex-induced vibration of twin tandem circular cylinders at low Reynolds number. In the numerical simulations, the upstream cylinder is fixed, while the rear cylinder is allowed to freely oscillate in both transverse and stream-wise directions. The damping ratio 0.007 and the reduced velocity varying from 3.0 to 12.0 are used for the computations. The effects of the in-line center-to-center distance (LX / D = 3.0、5.0、8.0) and the mass ratio (m* = 5.0、10.0、20.0) on the displacement and forces of the downstream cylinder are examined. The numerical simulations indicate that the spacing ratio (LX / D) has great influence on the VIV responses in terms of the lock-in band, displacement and fluid forces. Several flow modes under different spacing ratios and reduced velocities are observed, which are closely related to the VIV responses. It was also found that the change of the mass ratio may give rise to different characteristics of VIV responses for the twin cylinders in tandem at low Reynolds number.