Device and method of handling a radio access technology tight interworking of two cellular systems

A communication device for handling a radio access technology (RAT) tight interworking, comprising:

• a storage device;
• and
• a processing circuit, coupled to the storage device, wherein the storage device stores, and the processing circuit is configured to execute instructions of:
• connecting to a first base station (BS) via a first RAT;
• receiving a message from the first BS via the first RAT;
• determining whether the message configures the communication device to perform RAT tight interworking, or to perform an inter-RAT handover from the first RAT to a second RAT;
• transmitting, upon determining that the message configures the communication device to perform RAT tight interworking, a first response message to the first BS via the first RAT in response to the message, connecting to a second BS via the second RAT in response to the message, transmitting a second response message via the second RAT to the second BS in response to the message, and communicating second data via the second RAT with the second BS while simultaneously communicating first data via the first RAT with the first BS;
• and
• disconnecting, upon determining that the message configures the communication device to perform the inter-RAT handover from the first RAT to the second RAT, from the first RAT, connecting to the second BS via the second RAT in response to the message, transmitting a third response message to the second BS via the second RAT in response to the message and communicating third data via the second RAT with the second BS, and not transmitting the first response message to the first BS.