Different numbers of sub-carriers can be assigned to different users, in view to support differentiated Quality of Service (QoS), i.e. to control the data rate and error probability individually for each user. The aim of the paper lies in maximizing the system total throughput by optimall coordinating the relay and subcarrier assignment among the multiple pairs of two-way users. In this Paper, consider an OFDM-based network where multiple relays help multiple pairs of source nodes to conduct Bidirectional communications. The joint optimization problem of subcarrier pairing based subcarrier assignment and relay selection for multiple two-way users is considered as a combinatorial optimization problem.