Abstract: For sensor networks, maximizing the network lifetime is the main concern. Since sensor networks are mainly designed to cooperate on some joint task, the design intention is to minimize the total energy consumption in the network instead of minimizing energy consumption of individual nodes. In repetition-based cooperative diversity schemes, each relay requires its own subchannel for repetition. Therefore, full spatial diversity benefit comes at a price of decreasing bandwidth efficiency with the number of cooperating relays. Moreover, total circuit energy consumption also increases with the number of active relays. Especially in short-range applications, the circuit energy consumption is comparable to or even dominates the transmission energy. A simplified technique to achieve spatial diversity in cooperative networks is the opportunistic relaying scheme. The basic idea is to choose the best relay node amongst all available relays through a certain policy. On the other hand, energy and spectral efficiency can be enhanced by employing adaptive transmission techniques, which could fully utilize the time-varying wireless channels by adjusting transmission parameters, such as transmission power and constellation size. In this work, we propose a relay selection scheme to minimize the total energy consumption, which includes both the transmission energy and the circuit energy consumption. The energy efficiency for orthogonal opportunistic decode-and-forward (DF) relaying is improved by optimizing transmit power, rate and relay selection jointly.The proposed scheme allows source and relay nodes to transmit information using different transmit power. Given the target bit-error-rate (BER) and system parameters, the optimization problem involving transmit powers, modulation formats and the relay selection is solved in a distributed manner. The numerical results indicate the proposed scheme can achieve more energy saving than the cooperative MIMO transmission and reception.