Interworking of Wireless Local Area Network (WLAN) and 3G mobile network (3G) provides ubiquitous wireless communication at high data rates and mobility condition. Using dual mode User Equipment (UE), Subscribers can access large different services across a wide range of propagation environment with a variety of QoS (Quality of Services) requirements. In this paper we describe six different scenarios of WLAN and 3G interworking proposed by 3GPP (3rd Generation Partnership Project) Release 6. In addition, we illustrate the architecture, interfaces and new entities of interworking scenario 3. The security as well as the applied protocols supporting for scenario 3 are addressed. Finally, the user traffic routing and the procedure of tunnel establishment are also discussed.