In order to make good use of the limited energy, ant colony optimization (ACO) was applied to inter-cluster routing mechanism. An uneven clustering routing algorithm for Wireless Sensor Networks (WSNs) based on ant colony optimization (ACO) was proposed. The algorithm utilized the dynamic adaptability and optimization capabilities of the ant colony to get the optimum route between the cluster head. Meanwhile, it organized different cluter in different size based on the distance between cluster heads and sink node, and clusters closer to sink had smaller sizes than those farther away from the sink, thus the closer cluster heads could preserve energy for the inter-cluster data forwarding. Simulation result indicates that the algorithm effectively balances the network energy consumption and prolongs the network life cycle compared with LEACH and PARA.