Our main aim is to develop and demonstrate energy saving technologies for mobile terminals (MTs) equipped with multiple wireless interfaces. We propose a new framework of device-to-device (D2D) cooperative virtual small-cell network in which a MT acts a virtual base station to take advantage of the good channel quality of long range link and the low energy consuming interface of short range link to save energy in MTs. In the proposed D2D cooperative virtual small-cell framework, how to select proxy MT which connects to the wide area network (WAN) and provides the local area network (LAN) connection to cooperated MTs is an important issue, and it is influenced by multiple factors such as channel quality both in WAN and LAN, throughput, battery lifetime, mobility of MTs. In this paper, the energy efficiency improvement is evaluated by a smartphone test-bed and the impacts of WAN channel condition and the number of cooperated MTs are discussed. The result shows that up to 66% improvement can be achieved by the proposal compared to the traditional systems without cooperation.