A method for handling data transmission by a mobile station with a first RF module and second RF module is provided. The mobile station is coupled to a computer acquiring an IP address IP(U). A packet with a source IP address IP(U) and a destination IP address IP(Y) corresponding to an access network is received by the mobile station. The first RF module is assigned to transmit the received packet by replacing the source IP address IP(U) with a source IP address IP(A). Another packet with the source IP address IP(U) and destination IP address IP(Y) is assigned to the second RF module, and the source IP address IP(U) is replaced by a source IP address IP(B). The replaced packets are transmitted to the access network via the first and second RF module respectively.