A first communication device for handling a PDCP operation comprises at least one storage device for storing instructions and at least one processing circuit coupled to the at least one storage device. The at least one processing circuit is configured to execute the instructions stored in the at least one storage device. The instructions comprise receiving a first plurality of PDCP Data 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; updating the RX_NEXT, the RX_DELIV and the RX_REORD; performing a PDCP reestablishment; setting the RX_NEXT, the RX_DELIV and the RX_REORD to a plurality of initial values; receiving a second PDCP Data PDU associated to the RB, from the second communication device; and processing the second PDCP Data PDU according to the RX_NEXT, the RX_DELIV and the RX_REORD.