The disclosure is directed to a connection modification method in a dual connectivity scenario. In one of the exemplary embodiments, a UE configures a first MCG bearer for communication with a first base station; configures a SCG bearer for communication with a second base station; generates and transmits a first encrypted PDCP SDU by encrypting a first PDCP SDU of the first MCG bearer by using a first security key, a first TX_HFN and a first PDCN SN associated with the first PDCP SDU; generates and transmits a second encrypted PDCP SDU; receives a third bearer configuration re-configuring the SCG bearer as either a second MCG bearer or a split bearer; generates a third encrypted PDCP SDU by encrypting a third PDCP SDU of the SCG bearer and transmits the third encrypted PDCP SDU after the SCG bearer is reconfigured as the second MCG bearer or the split bearer.