A method and a system are disclosed for transmitting/receiving data in heterogeneous communication systems having a mobile station, a first communication system currently accessed by the mobile station, and a second communication system having an access scheme different from the first communication system. The method includes accessing, by the mobile station, the first communication system and maintaining an active session with the first communication system; accessing, by the mobile station, the second communication system and requesting an active session to the second communication system when sensing handover to the second communication system while the active session with the first communication system is maintained; and maintaining, by the mobile station, the active session with the first communication system by means of an active session with the second communication system created according to the requested active session.