In this letter, a novel association method called ELBA (efficient load balancing association) is proposed for improved load balancing in IEEE 802.15.4-based WSNs (wireless sensor networks). ELBA adds new nodes to the network in an efficient load-balancing manner by exploiting not only RSSI (received signal strength indicator), which is used in the standard, but also traffic-load, the number of allocated GTSs (guaranteed time slots), and the number of parent nodes and child nodes. Simulation results show that ELBA offers better performance in load balancing and preventing congestion.