Mass ratio is an important parameter that influences the vortex- induced vibration of circular cylinder, especially for a two degree of freedom system. Numerical simulations have been performed for two equal-diameter cylinders with different mass ratios in tandem arrangement which are subjected to uniform flows in subcritical flow regime. The distances between the upstream and downstream cylinders were three and four times of the cylinder diameter. The purpose of this study is to investigate the effect of upstream cylinder's mass ratio on the vibration of downstream cylinder. The shear stress transport detached eddy turbulence model has been used for simulating the turbulent flow around the two cylinders. The numerical results of a single cylinder subjected to 2DOF vibration have been compared with the experimental results available for such cases to validate the present study.