Energy conservation, while ensuring an adequate level of service, is a major concern in Low power and Lossy Networks (LLNs), because the nodes are typically deployed and are not replaced in case of failure. Several efforts have recently led to the standardization of a routing protocol for LLNs. The standard provides several criteria that can be used as a routing metric. The working group RoLL of the IETF developed a routing protocol for 6LoWPAN sensor network (IPv6 over IEEE 802.15.4) (Ko et

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... , 2011), RPL, recently standardized. Using this protocol could become common and standard in IPv6 sensor networks in the future. Most implementation of the protocol makes use of the transmission rate successfully (ETX) as metric and focuses on the reliability of links. In this paper we present the use of the residual energy and the transmission delay as routing metric in the next hop selection process for the RPL protocol. We design an objective function for this metric based on ant colony optimization (ACO), and then we compare the results of experiments realized with the RPL based on ETX.