3GPP 43.318 specifies the Generic Access Network (GAN) for interworking between Wireless Local Area Network (WLAN) and Universal Mobile Telecommunications System (UMTS) core network. A dual-mode Mobile Station (MS) is equipped with two communication modules to support both WLAN and UMTS radio technologies, which shortens the battery lifetime of the MS. This paper proposes an analytical model and conducts simulation experiments to study the power consumption of dual-mode MSs in terms of the power consumption indicator and mean packet waiting time. Our study provides guidelines for designing WLANUMTS dual-mode MSs.