A system for handling packet-switched data transmission is provided. The system includes a first RF module, a second RF module, a first subscriber identity card camping on a cell via the first RF module, a second subscriber identity card camping on the same cell or a different cell via the second RF module and a load balancing unit. The load balancing unit receives an uplink IP packet, determines one subscriber identity card from the first and second subscriber identity cards when the uplink IP packet comprises information regarding an unestablished connection of a recognized application, and transmits the uplink IP packet to a destination via the determined subscriber identity card and the RF module corresponding to the determined subscriber identity card.