A first communication device for handling a Packet Data Convergence Protocol, PDCP, operation in a 3GPP wireless communication 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: receiving a first plurality of PDCP Data Protocol Data Units, PDUs, associated to a radio bearer, RB, from a second communication device; processing the first plurality of PDCP Data PDUs according to a RX_NEXT, a RX_DELIV and a RX_REORD which are associated to the RB; updating the RX_NEXT, the RX_DELIV and the RX_REORD, when processing the first plurality of PDCP Data PDUs; performing a PDCP reestablishment, after updating the RX_NEXT, the RX_DELIV and the RX_REORD; characterised by: setting the RX_NEXT, the RX_DELIV and the RX_REORD to a plurality of initial values in response to the PDCP reestablishment, wherein the initial values for the RX_NEXT and for the RX_DELIV are 0; receiving a second PDCP Data PDU associated to the RB, from the second communication device, after the PDCP reestablishment; and processing the second PDCP Data PDU according to the RX_NEXT, the RX_DELIV and the RX_REORD, after setting the RX_NEXT, the RX_DELIV and the RX_REORD to the plurality of initial values. 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: receiving a first plurality of PDCP Data Protocol Data Units, PDUs, associated to a radio bearer, RB, from a second communication device; processing the first plurality of PDCP Data PDUs according to a RX_NEXT, a RX_DELIV and a RX_REORD which are associated to the RB; updating the RX_NEXT, the RX_DELIV and the RX_REORD, when processing the first plurality of PDCP Data PDUs; performing a PDCP reestablishment, after updating the RX_NEXT, the RX_DELIV and the RX_REORD; receiving a first plurality of PDCP Data Protocol Data Units, PDUs, associated to a radio bearer, RB, from a second communication device; processing the first plurality of PDCP Data PDUs according to a RX_NEXT, a RX_DELIV and a RX_REORD which are associated to the RB; updating the RX_NEXT, the RX_DELIV and the RX_REORD, when processing the first plurality of PDCP Data PDUs; performing a PDCP reestablishment, after updating the RX_NEXT, the RX_DELIV and the RX_REORD; characterised by: setting the RX_NEXT, the RX_DELIV and the RX_REORD to a plurality of initial values in response to the PDCP reestablishment, wherein the initial values for the RX_NEXT and for the RX_DELIV are 0; receiving a second PDCP Data PDU associated to the RB, from the second communication device, after the PDCP reestablishment; and processing the second PDCP Data PDU according to the RX_NEXT, the RX_DELIV and the RX_REORD, after setting the RX_NEXT, the RX_DELIV and the RX_REORD to the plurality of initial values. setting the RX_NEXT, the RX_DELIV and the RX_REORD to a plurality of initial values in response to the PDCP reestablishment, wherein the initial values for the RX_NEXT and for the RX_DELIV are 0; receiving a second PDCP Data PDU associated to the RB, from the second communication device, after the PDCP reestablishment; and processing the second PDCP Data PDU according to the RX_NEXT, the RX_DELIV and the RX_REORD, after setting the RX_NEXT, the RX_DELIV and the RX_REORD to the plurality of initial values.