A communication device for handling a first radio access technology, RAT, and second RAT tight interworking, wherein the first RAT is a long term evolution, LTE, RAT and the second RAT is a new radio, NR, RAT of a fifth generation, 5G, communication technology, or the first RAT is a NR RAT of a 5G communication technology and the second RAT is a LTE RAT, the communication device comprising: a storage device, for storing instructions of: connecting to a first base station, BS, via a first RAT; receiving a first message of the first RAT from the first BS via the first RAT, wherein the first message comprises a second message of a second RAT and a first transaction identifier, TI, and the second message is adapted to configure the communication device to connect to a second BS via the second RAT and comprises a second TI; transmitting a first response message to the first BS via the first RAT in response to the first message, wherein the first response message comprises the first TI and a second response message, and the second response message comprises the second TI; connecting to the second BS via the second RAT in response to the second message; and communicating second data via the second RAT with the second BS according to a configuration of the second RAT in the second message, when communicating first data via the first RAT with the first BS; and a processing circuit, coupled to the storage device, configured to execute the instructions stored in the storage device. a storage device, for storing instructions of: connecting to a first base station, BS, via a first RAT; receiving a first message of the first RAT from the first BS via the first RAT, wherein the first message comprises a second message of a second RAT and a first transaction identifier, TI, and the second message is adapted to configure the communication device to connect to a second BS via the second RAT and comprises a second TI; transmitting a first response message to the first BS via the first RAT in response to the first message, wherein the first response message comprises the first TI and a second response message, and the second response message comprises the second TI; connecting to the second BS via the second RAT in response to the second message; and communicating second data via the second RAT with the second BS according to a configuration of the second RAT in the second message, when communicating first data via the first RAT with the first BS; and connecting to a first base station, BS, via a first RAT; receiving a first message of the first RAT from the first BS via the first RAT, wherein the first message comprises a second message of a second RAT and a first transaction identifier, TI, and the second message is adapted to configure the communication device to connect to a second BS via the second RAT and comprises a second TI; transmitting a first response message to the first BS via the first RAT in response to the first message, wherein the first response message comprises the first TI and a second response message, and the second response message comprises the second TI; connecting to the second BS via the second RAT in response to the second message; and communicating second data via the second RAT with the second BS according to a configuration of the second RAT in the second message, when communicating first data via the first RAT with the first BS; and a processing circuit, coupled to the storage device, configured to execute the instructions stored in the storage device.