Device and method of handling data transmissions in a wireless communication system

A first base station (BS) in a network for communicating with a communication device according to a Cellular network/wireless local area network (WLAN) Radio Level Integration with Internet Protocol Security (IPsec) Tunnel (CWIP), comprising:

• a storage device, for storing instructions;
• and
• a processing circuit, coupled to the storage device, wherein the processing circuit is configured to execute the instructions of:
• transmitting a CWIP configuration and a CWIP aggregation configuration to the communication device;
• transmitting a plurality of CWIP Encapsulation Protocol (CWIPEP) protocol data units (PDUs) over a radio bearer (RB) via a cellular connection or a WLAN to the communication device, after transmitting the CWIP configuration and the CWIP aggregation configuration;
• transmitting a Handover Request message to a second BS, to request the second BS to prepare a handover for the communication device;
• receiving a Handover Request Acknowledge message from the second BS, wherein the Handover Request Acknowledge message comprises a Handover Command message indicating the communication device to perform the handover to a cell of the second BS;
• transmitting the Handover Command message to the communication device;
• and
• forwarding an IP packet associated to the RB to the second BS in response to the handover, wherein the IP packet is comprised in a CWIPEP PDU and the CWIPEP PDU is not transmitted successfully to the communication device before the communication device performs the handover.