A two-body rolling wave energy conversion system (WECS) is proposed. The system utilizes the rolling difference between the upper buoy and the lower one. On the basis of the lower buoy’s stability, the rolling of the upper buoy determines the wave energy absorption. The buoy's rolling is determined by its natural frequency, which is mainly affected by the G.C position arrangement. Therefore, numerical simulation is applied. A box-shaped buoy is selected as the upper buoy, and the system working in long crest wave with ten conditions is analyzed, in order to find out how the G.C position arrangement influences the wave energy absorption and the PTO damping selection. The results show that G.C position arrangement does affect the wave energy absorption by changing the natural frequency, and a lower G.C. position can enlarge the frequency range, and can absorb more wave energy.