A communication device for handling data transmission/reception for dual connectivity comprises a storage unit for storing instructions of connecting to a first base station (BS) and a second BS; receiving a message from the first BS, wherein the message configures a handover to a third BS and a connection change to a fourth BS; updating a first parent key to a first updated parent key and updating a second parent key to a second updated parent key based on the first updated patent key, in response to the message; updating a first security key to a first updated security key based on the first updated patent key and updating a second security key to a second updated security key based on the second updated parent key, in response to the message.