Due to the proliferation of smart mobile devices and the developments in wireless communication, mobile ad hoc networks (MANETs) are gaining more and more attention in recent years. Routing in MANETs is a challenge, especially when the network contains a large number of nodes. The clustering technique is a popular method to organize the nodes in MANETs. It divides the network into several clusters and assigns a cluster head to each cluster for intra- and inter-cluster communication. Clustering is NP-hard and needs to consider multiple objectives. In this paper we propose a Pareto-based ant colony optimization (ACO) algorithm to deal with this multiobjective optimization problem. A new encoding scheme is proposed to reduce the size of search space, and a new decoding scheme is proposed to generate high-quality solutions effectively. Experimental results show that our approach is better than several benchmark approaches.