A heterogeneous radio access network system includes a mobile device and a first base station operated in a first wireless access network. The mobile device is wireless communicated with the first base station. One of the mobile device and the first base station sends a traffic steering initial message to request a traffic steering operation, the traffic steering initial message including a plurality of traffic flows. The other one of the mobile device and the first base station decides whether to accept or reject the traffic steering initial message. If the other one of the mobile device and the first base station accepts the traffic steering initial message, the mobile device performs a selective traffic steering, wherein the selective traffic steering comprises traffic flow(s), and the traffic flow(s) in the selective traffic steering are selected among the traffic flows of the traffic steering initial message.