A Medium Access Control (MAC) apparatus and method for use in a channel overlay network of a wireless access system. The wireless access system, which is based on a multi-carrier, includes a plurality of physical modules for receiving signals from a plurality of terminals, a plurality of MAC (Medium Access Control) modules for processing the signals received via the physical modules while being classified according to predetermined traffic classes, and a controller connected to the MAC modules for integratedly managing the MAC modules. The MAC apparatus includes an additional MAC module used for a control channel and a plurality of MAC modules for supporting various QoS traffic data according to individual classes, thereby improving scheduling complexity in a MAC layer.