In this letter, we first analyze the delay of the WLAN discovery scheme specified for 3GPP and WLAN interworking networks. Theoretical analysis indicates that the delay of the discovery scheme given by 3GPP increases linearly with the number of WLAN channels needed to be scanned. To reduce the discovery delay, we then propose an effective WLAN discovery scheme utilizing the cellular network to aid in the broadcasting the information of WLANs. Thus the number of WLAN channels required to be scanned for users is greatly reduced. The effectiveness of the proposed scheme is demonstrated by analysis and simulation.