A Wireless Sensor Network (WSN) is a network of thousands of low-cost sensors which have a limited operating lifetime. A sensor node generally comprises four basic elements, including a transceiver unit, a detection unit, a processing unit, and an energy supply unit. One of the major challenges in the WSN is to increase the network life-time. Since the transmitting/receiving unit has the highest energy consumption, reducing it increases the network life-time significantly. In this work, we propose to differentiate resources or services in order to provide high performances of Quality of Service (QoS) to the prioritized node, since the Medium Access Control (MAC) layer is considered as the responsible that manages the access to the transmission channel. Therefore, we need to manage the quality of service of this layer in order to give high chance to nodes with a lower energy level to get access to the tranmission channel to send the data before their batteries run out. In this article, we present a new technique called Energy based Collision Avoidance (ECA-MAC), that handles WSN energy constraints and the QoS designed for versatile applications. ECA-MAC protocol aims at controlling the access to the medium according to nodes energy level and at reducing the collision using different Contention Windows (CW). Our simulation results show that ECA-MAC protocol improves the QoS in terms of Packet Delivery Ratio (PDR), message latency, and energy consumption.