A method and system are provided for servicing a full duplex call in Push-To-Talk over cellular (PoC) in a communication system including at least two or more calling/called terminals for receiving a Push-To-Talk (PTT) service and a mobile network for providing the PTT service to the calling/called terminals. The calling terminal generates a full-duplex mode request message and transmits the full-duplex mode request message to the mobile network. The mobile network transmits the full-duplex mode request message to a PoC server that provides a PoC service to the calling terminal. Upon receiving the full-duplex mode request message, a PoC server of the calling terminal sets the calling terminal to a full duplex mode, and transmits the full-duplex mode request message to a PoC server of the called terminal. The PoC server of the called terminal transmits the full-duplex mode request message to the called terminal, and performs a call between the calling terminal and the called terminal in the full duplex mode if the called terminal responds to the full-duplex mode request message.