To solve load balancing problem of Power Line Communication (PLC) network in Smart Grid (SG), an adaptive opportunistic routing protocol named LBORP (Load Balancing Opportunistic Routing Protocol) was proposed. All the candidate forwarding nodes receiving a packet in LBORP had the opportunity to participate in packet forwarding. As a result, packet forwarding was no longer limited to one routing path, which avoided load imbalance caused by traffic with only one link for traffic to pass. And forwarding priority of the candidate forwarding nodes considered not only the distance from forwarding nodes to destination node, but also instability of PLC links and change of traffic. Besides, an implicit acknowledgment scheme was adopted in LBORP, further reducing the end-to-end delay of the proposed protocol. In simulation experiment, compared to PLC-TR (Power Line Communication-Tree Routing) and PLC-OR (Power Line Communication-Opportunistic Routing), LBORP reduced delay by 19.7% and 45.8% respectively and reduced packet loss rate by 23.4% and 32.5% respectively. Experimental results show that LBORP can achieve network load balancing, improve network reliability and reduce end-to-end delay.