A network device for establishing quality of service (QoS) between two terminal devices includes a transceiver module and a state-machine setting module. The transceiver module is configured for receiving establishing requests, request responses, acknowledge messages, and QoS requests from any one of the two terminal devices. The state-machine setting module is configured for setting a state of the network device according to a current state of the network device and messages received by the transceiver module, and the state of the network device includes an idle state, an inviting state, a trying state, an acknowledge state, and a QoS state.