A communication device for handling data transmission, comprising: a storage device, for storing instructions of: configuring a first bearer and a second bearer according to at least one bearer configuration received from a base station; 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 first radio resource control, RRC, message from the base station before transmitting the first encrypted packet to the base station successfully, wherein the first RRC message configures the communication device to transmit at least one packet of the first flow via the second bearer to the base station; transmitting the first encrypted packet to the base station via the first bearer, after receiving the first RRC message; encrypting a second 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 first RRC message; and transmitting the second encrypted packet to the base station via the second bearer; 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: configuring a first bearer and a second bearer according to at least one bearer configuration received from a base station; 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 first radio resource control, RRC, message from the base station before transmitting the first encrypted packet to the base station successfully, wherein the first RRC message configures the communication device to transmit at least one packet of the first flow via the second bearer to the base station; transmitting the first encrypted packet to the base station via the first bearer, after receiving the first RRC message; encrypting a second 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 first RRC message; and transmitting the second encrypted packet to the base station via the second bearer; and configuring a first bearer and a second bearer according to at least one bearer configuration received from a base station; 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 first radio resource control, RRC, message from the base station before transmitting the first encrypted packet to the base station successfully, wherein the first RRC message configures the communication device to transmit at least one packet of the first flow via the second bearer to the base station; transmitting the first encrypted packet to the base station via the first bearer, after receiving the first RRC message; encrypting a second 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 first RRC message; and transmitting the second encrypted packet to the base station via the second bearer; and a processing circuit, coupled to the storage device, configured to execute the instructions stored in the storage device.