A communication device for handling data transmissions after a handover, comprising: a storage device, for storing instructions of: receiving a first radio resource control, RRC, message configuring a first data radio bearer, DRB, and configuring a mapping that a first flow is mapped to the first DRB, from a first base station, BS; transmitting a first plurality of Protocol Data Units, PDUs, of the first flow over the first DRB in response to the first RRC message; receiving a second PDU of the first flow or a second flow over a second DRB from the first BS; transmitting a third plurality of PDUs of the first flow over the second DRB in response to the second PDU; receiving a handover command from the first BS; performing a handover to a second BS in response to the handover command; keeping a DRB configuration configuring the first DRB in response to the handover; and transmitting a fourth plurality of PDUs of the first flow over the second DRB to the second BS in response to the handover command, after performing the handover to the second 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: receiving a first radio resource control, RRC, message configuring a first data radio bearer, DRB, and configuring a mapping that a first flow is mapped to the first DRB, from a first base station, BS; transmitting a first plurality of Protocol Data Units, PDUs, of the first flow over the first DRB in response to the first RRC message; receiving a second PDU of the first flow or a second flow over a second DRB from the first BS; transmitting a third plurality of PDUs of the first flow over the second DRB in response to the second PDU; receiving a handover command from the first BS; performing a handover to a second BS in response to the handover command; keeping a DRB configuration configuring the first DRB in response to the handover; and transmitting a fourth plurality of PDUs of the first flow over the second DRB to the second BS in response to the handover command, after performing the handover to the second BS; and receiving a first radio resource control, RRC, message configuring a first data radio bearer, DRB, and configuring a mapping that a first flow is mapped to the first DRB, from a first base station, BS; transmitting a first plurality of Protocol Data Units, PDUs, of the first flow over the first DRB in response to the first RRC message; receiving a second PDU of the first flow or a second flow over a second DRB from the first BS; transmitting a third plurality of PDUs of the first flow over the second DRB in response to the second PDU; receiving a handover command from the first BS; performing a handover to a second BS in response to the handover command; keeping a DRB configuration configuring the first DRB in response to the handover; and transmitting a fourth plurality of PDUs of the first flow over the second DRB to the second BS in response to the handover command, after performing the handover to the second BS; and a processing circuit, coupled to the storage device, configured to execute the instructions stored in the storage device.