This paper presents a new load balancing method for wireless sensor networks. In most routing schemes a sender forwards its packets destined for the same sink through the same neighbor. Thus, the nodes located in the intersection of multiple paths carry a large amount of workload and exhaust their energy rapidly. To overcome this problem, we propose a proportional load balancing method (PLB). In PLB, a sender chooses multiple paths for a destination (sink) and forwards its packets over these paths. By spreading the load over multiple paths, PLB prolong overall network life time, increases network efficiency and prevent fluctuation problem in load balancing. The experimental results verify the achievements of PLB compared to the previous studies.