In this paper, we present the experimental measurements of real-time voice packets over 3G-WLAN interworking system. It is of interest to evaluate (i) the performance of VoIP with IPSec tunnel in 3G-WLAN interworking system, (ii) maximum number of VoIP connections in a single 802.11b Access Point(AP), (iii) handoff latency due to terminal mobility and (iv) impact of terminal mobility on the VoIP performance. Our results show that the performance can degrade significantly with the increase of VoIP connections and with the terminal mobility. Furthermore, the performance degradation affects all the existing VoIP connections, not merely the new connection or the UE which undergoes mobility. The amount of delay during terminal movement severely affects the performance of all real-time applications in that particular AP. © 2005 IEEE.