As the demands for various multimedia services requiring high capacity increase, shortage of wireless resources becomes a serious problem in cellular networks. In order to reduce the heavy load and increase overall capacity of cellular networks, internetworking between WLANs and cellular networks has being considered. However, in the internetworking of cellular networks and WLANs, since performance of overall networks is closely related with the execution timing of vertical handover, an efficient vertical handover triggering mechanism is required. Thus, in this paper, we propose a data rate based vertical handover triggering mechanism (DR-HTM) based on IEEE 802.21 Media Independent Handover (MIH) standard in order to maximize capacity of both WLANs and cellular networks. In DR-HTM, whenever a mobile node discovers WLAN, it obtains achievable data rate of the WLAN by using remote MIH services. Based on the information, the mobile node determines execution of vertical handover. To evaluate the performance of the DR-HTM, we have performed intensive simulations for handover triggering mechanisms based on data rate and RSS. From the results, we can find that loads of cellular networks are effectively shared with WLANs and capacity of overall networks can be maximized by using DR-HTM.