Cooperative communication is a promising technique which tends to change the traditional transmission scheme in wireless networks. It can improve the performance of the network in wireless environment. This paper proposes a cooperative MAC protocol using active relays in wireless sensor networks called TEC-MAC. This protocol enables remote nodes to send their data by using intermediate nodes as relays (or helpers) to their Access Point (AP). This proposed protocol is performed and evaluated via analytical analysis. Analytical results show that the new protocol significantly improves the network performance in terms of throughput and energy efficiency compared with a Busy Tone based cooperative MAC protocol (BTAC), and IEEE802.11b Medium Access Control (MAC) protocols.