In this paper, we propose an efficient routing algorithm for large scale cluster-based wireless sensor networks. The technique uses two routing levels. In the first level (intra-cluster), cluster members send data directly to their cluster head. In the second level (inter-cluster), the cluster heads use ant colony optimization (ACO) algorithm, which is a biologically inspired paradigm for optimization approach, to find a route to the base station. As only cluster heads participate in the inter-cluster routing operation, the method can provide a smooth operation more effectively. The delay of the algorithm is minimized by using the ant colony optimization algorithm along with clustering. To assess the efficiency of the proposed algorithm, we compare the method with some previous routing algorithms. The results show lower power consumption and more load balancing.