Device and method of handling data transmission

A communication device of handling data transmission, comprising:

• a storage device;
• and
• a processing circuit, coupled to the storage device, wherein the storage device stores instructions, and the processing circuit is configured to execute the instructions of:
• configuring a first bearer and a second bearer according to at least one bearer configuration received from a network;
• encrypting a first packet of a first flow into a first encrypted packet according to an encryption key and a first bearer identity of the first bearer;
• receiving a second packet of the first flow from the network via the second bearer, before transmitting the first encrypted packet to the network successfully;
• transmitting the first encrypted packet to the network via the first bearer, after receiving the second packet;
• encrypting a third packet of the first flow into a second encrypted packet according to the encryption key and a second bearer identity of the second bearer in response to the second packet;
• and
• transmitting the second encrypted packet to the network via the second bearer.