RPL is the routing protocol for low-power and lossy networks and has recently been considered as the standard routing protocol for IPv6 based wireless sensor networks using the IEEE 802.15.4 protocol. RPL is oriented towards supporting multi-point to point communications in which multiple points typically communicate with one sink node that considered the root of the RPL tree. RPL aims to minimize the cost to reach the sink node from any node in the network using an objective function. However, RPL lacks a load balancing mechanism to maximize the lifetime of the battery-powered wireless sensor networks by preventing overloaded nodes from being drained quickly. In this paper, we propose the Minimum Degree RPL (MD-RPL) which builds a minimum degree spanning tree to enable load balancing in RPL. MD-RPL modifies the original tree formed by RPL to decrease its degree. Extensive simulations were carried out to evaluate the proposed scheme using the COOJA simulator. Our MD-RPL results in up to 15.6% improvement in the maximum power consumed which implies an improvement in the lifetime of the network.