A method for robust moving between a roaming Third Generation Partnership Project (3GPP) network and a non-3GPP network is provided. The method includes the steps of: connecting with a home 3GPP network over one of a roaming 3GPP network and a non-3GPP network by a User Equipment (UE); performing a first initial attach procedure with the home 3GPP network over the other of the roaming 3GPP network and the non-3GPP network by the UE in response to the UE moving from the one of the roaming 3GPP network and the non-3GPP network to the other of the roaming 3GPP network and the non-3GPP network; and keeping the connection over the one of the roaming 3GPP network and the non-3GPP network by the UE during the first initial attach procedure over the other of the roaming 3GPP network and the non-3GPP network.