A communication device of handling a communication with a network, including a cellular network and a wireless local area network, WLAN, the communication device comprising: a storage unit for storing instructions; a processing means, coupled to the storage unit, wherein the storage unit stores, and the processing means is configured to execute the instructions of: receiving a radio resource control, RRC, message, configuring cellular-WLAN aggregation, CWA, to the communication device, from a base station, BS, of the cellular network; deriving a first pairwise master key, PMK, according to the RRC message; deriving a first encryption key from the first PMK for encrypting a first data transmitted to the WLAN or decrypting a second data received from the WLAN using the first PMK; said device characterised by: releasing the CWA during connecting to the WLAN; performing an extensible authentication protocol, EAP, authentication and key agreement, AKA, procedure with the WLAN to derive a second PMK, when releasing the CWA; and deriving a second encryption key from the second PMK for encrypting a third data transmitted to the WLAN or decrypting a fourth data received from the WLAN using the second PMK. a storage unit for storing instructions; a processing means, coupled to the storage unit, wherein the storage unit stores, and the processing means is configured to execute the instructions of: receiving a radio resource control, RRC, message, configuring cellular-WLAN aggregation, CWA, to the communication device, from a base station, BS, of the cellular network; deriving a first pairwise master key, PMK, according to the RRC message; deriving a first encryption key from the first PMK for encrypting a first data transmitted to the WLAN or decrypting a second data received from the WLAN using the first PMK; said device characterised by: releasing the CWA during connecting to the WLAN; performing an extensible authentication protocol, EAP, authentication and key agreement, AKA, procedure with the WLAN to derive a second PMK, when releasing the CWA; and deriving a second encryption key from the second PMK for encrypting a third data transmitted to the WLAN or decrypting a fourth data received from the WLAN using the second PMK. receiving a radio resource control, RRC, message, configuring cellular-WLAN aggregation, CWA, to the communication device, from a base station, BS, of the cellular network; deriving a first pairwise master key, PMK, according to the RRC message; deriving a first encryption key from the first PMK for encrypting a first data transmitted to the WLAN or decrypting a second data received from the WLAN using the first PMK; said device characterised by: releasing the CWA during connecting to the WLAN; performing an extensible authentication protocol, EAP, authentication and key agreement, AKA, procedure with the WLAN to derive a second PMK, when releasing the CWA; and deriving a second encryption key from the second PMK for encrypting a third data transmitted to the WLAN or decrypting a fourth data received from the WLAN using the second PMK. releasing the CWA during connecting to the WLAN; performing an extensible authentication protocol, EAP, authentication and key agreement, AKA, procedure with the WLAN to derive a second PMK, when releasing the CWA; and deriving a second encryption key from the second PMK for encrypting a third data transmitted to the WLAN or decrypting a fourth data received from the WLAN using the second PMK.