A method of handling small data transmission for a core network is disclosed, wherein the core network comprises a data base, a network gateway node and a network control node. The method comprises the network gateway node receiving a service request message from a machine-type communication (MTC) server, wherein information of the service request message is eligible for the small data transmission between a mobile device and the network gateway node; and the network gateway node including a small data transmission indicator in the service request message, wherein the small data transmission indicator indicates the small data transmission.