A system comprising a first base station, BS, and a second BS for handling a handover in accordance with a 3GPP communication network, the system comprising: at least one storage device; and at least one processing circuit, coupled to the at least one storage device, wherein the at least one storage device stores, and the at least one processing circuit is configured to execute instructions of: the first BS transmitting a first Radio Resource Control, RRC, message on a signaling radio bearer, SRB, to a communication device, wherein the first RRC message configures a data radio bearer, DRB; the first BS receiving a first RRC response message from the communication device on the SRB, in response to the first RRC message; the first BS receiving a first plurality of Packet Data Convergence Protocol, PDCP, Service Data Units, SDUs, associated to the DRB from the communication device; the first BS processing the first plurality of PDCP SDUs according to a RX_NEXT, a RX_DELIV and a RX_REORD which are associated to the DRB; the first BS performing a handover preparation procedure with the second BS for the communication device, to hand over the communication device to the second BS; the first BS transmitting a second RRC message on the SRB to the communication device in response to the handover preparation procedure; the second BS receiving a second RRC response message responding to the second RRC message on the SRB from the communication device; the first BS transmitting at least one of the RX_NEXT, the RX_DELIV and the RX_REORD to the second BS; the second BS receiving a second plurality of PDCP SDUs associated to the DRB from the communication device; and the second BS processing the second plurality of PDCP SDUs according to the at least one of the RX_NEXT, the RX_DELIV and the RX_REORD. at least one storage device; and at least one processing circuit, coupled to the at least one storage device, wherein the at least one storage device stores, and the at least one processing circuit is configured to execute instructions of: the first BS transmitting a first Radio Resource Control, RRC, message on a signaling radio bearer, SRB, to a communication device, wherein the first RRC message configures a data radio bearer, DRB; the first BS receiving a first RRC response message from the communication device on the SRB, in response to the first RRC message; the first BS receiving a first plurality of Packet Data Convergence Protocol, PDCP, Service Data Units, SDUs, associated to the DRB from the communication device; the first BS processing the first plurality of PDCP SDUs according to a RX_NEXT, a RX_DELIV and a RX_REORD which are associated to the DRB; the first BS performing a handover preparation procedure with the second BS for the communication device, to hand over the communication device to the second BS; the first BS transmitting a second RRC message on the SRB to the communication device in response to the handover preparation procedure; the second BS receiving a second RRC response message responding to the second RRC message on the SRB from the communication device; the first BS transmitting at least one of the RX_NEXT, the RX_DELIV and the RX_REORD to the second BS; the second BS receiving a second plurality of PDCP SDUs associated to the DRB from the communication device; and the second BS processing the second plurality of PDCP SDUs according to the at least one of the RX_NEXT, the RX_DELIV and the RX_REORD.