A wireless sensor network consisting of a large number of nodes with limited battery power should minimize energy consumption at each node to prolong the network lifetime. In this paper, we propose a routing protocol based on a balanced tree, called BATR (Balanced Aggregation Tree Routing), which uses near optimal minimal spanning tree for balancing the power consumptions over all nodes. Our key idea is that a near optimal data aggregation can be achieved in terms of network lifetime when power consumption at each node can be nicely balanced. Simulation results show that the BATR can lengten system lifetime as compared with the conventional data aggregation protocols.