A connection modification method applicable to a user equipment, UE, which is capable of dually connecting to a master evolved Node B, MeNB, and a secondary evolved Node B, SeNB, the method comprising: receiving a first bearer configuration configuring a first master cell group, MCG, bearer for communication with the MeNB from the MeNB; configuring a first radio bearer as the first MCG bearer according to the first bearer configuration; receiving a second bearer configuration configuring a secondary cell group, SCG, bearer for communication with the SeNB from the MeNB; configuring a second radio bearer as the SCG bearer according to the second bearer configuration; generating a first encrypted Packet Data Convergence Protocol, PDCP, service data unit, SDU, by encrypting a first PDCP SDU of the first radio bearer by using a first security key, being a key used for encrypting and decrypting data for communications between the MeNB and the UE, a first transmitter hyper frame number, TX_HFN, and a first PDCP sequence number, SN, associated with the first PDCP SDU; transmitting the first encrypted PDCP SDU to the MeNB; generating a second encrypted PDCP SDU by encrypting a second PDCP SDU of the second radio bearer by using a second security key, used for encrypting and decrypting data for communications between the SeNB and the UE, a second TX_HFN and a second PDCP SN associated with the second PDCP SDU; transmitting the second encrypted PDCP SDU to the SeNB; receiving a radio resource control, RRC, message comprising a third bearer configuration re-configuring the SCG bearer to be either a second MCG bearer or a split bearer from the MeNB; re-configuring the second radio bearer either as the second MCG bearer or the split bearer according to the third bearer configuration; generating a third encrypted PDCP SDU by encrypting a third PDCP SDU of the second radio bearer by using the first security key, the second TX_HFN and a third PDCP SN associated with the third PDCP SDU, after the second radio bearer is reconfigured to the second MCG bearer or the split bearer; and transmitting the third encrypted PDCP SDU to the MeNB. receiving a first bearer configuration configuring a first master cell group, MCG, bearer for communication with the MeNB from the MeNB; configuring a first radio bearer as the first MCG bearer according to the first bearer configuration; receiving a second bearer configuration configuring a secondary cell group, SCG, bearer for communication with the SeNB from the MeNB; configuring a second radio bearer as the SCG bearer according to the second bearer configuration; generating a first encrypted Packet Data Convergence Protocol, PDCP, service data unit, SDU, by encrypting a first PDCP SDU of the first radio bearer by using a first security key, being a key used for encrypting and decrypting data for communications between the MeNB and the UE, a first transmitter hyper frame number, TX_HFN, and a first PDCP sequence number, SN, associated with the first PDCP SDU; transmitting the first encrypted PDCP SDU to the MeNB; generating a second encrypted PDCP SDU by encrypting a second PDCP SDU of the second radio bearer by using a second security key, used for encrypting and decrypting data for communications between the SeNB and the UE, a second TX_HFN and a second PDCP SN associated with the second PDCP SDU; transmitting the second encrypted PDCP SDU to the SeNB; receiving a radio resource control, RRC, message comprising a third bearer configuration re-configuring the SCG bearer to be either a second MCG bearer or a split bearer from the MeNB; re-configuring the second radio bearer either as the second MCG bearer or the split bearer according to the third bearer configuration; generating a third encrypted PDCP SDU by encrypting a third PDCP SDU of the second radio bearer by using the first security key, the second TX_HFN and a third PDCP SN associated with the third PDCP SDU, after the second radio bearer is reconfigured to the second MCG bearer or the split bearer; and transmitting the third encrypted PDCP SDU to the MeNB.