In recent years, various types of ad hoc routing protocols have been studied in the mobile ad hoc networks. Specifically, the cluster-based hierarchical routing algorithms have been developed to increase the system performance. However, the significant overhead resulting from the formation of the cluster structure make it unsatisfactory to assist the design of routing algorithms, especially with small number of communication pairs in the network. In this paper, an On-demand Routing-based Clustering (ORC) protocol is developed in order to alleviate the excessive overhead induced from conventional cluster formation. The cluster structure is simultaneously established along with the construction of the on-demand routing path. Depending on the existing number of communication pair within the network, the clusters are adaptively formed, maintained, and disbanded. The effectiveness of the proposed ORC algorithm can be observed via the simulation results.