In order to ensure an efficient network control for mobile ad hoc networks, nodes are usually aggregated into clusters to provide a convenient framework. However, an important issue on how to properly assign channels over a multi-cluster structure has not been studied in depth. In this paper, we first propose a new clustering algorithm to facilitate subsequent channel assignment. Each cluster has one clusterhead. Between clusterheads, there are gateways that relay control information and help eliminate the hidden terminal problem. Then two dynamic channel assignment (DCA) strategies are designed to make the best use of available channels by taking advantage of the spatial reuse concept. One strategy tries approaching a compact-channel pattern so that the channel reuse efficiency can be enhanced. On the other hand, the other strategy tries reducing the control overhead on the channel assignment so that the users can have a lower power consumption. We evaluate our cluster structure via simulation in terms of cluster stability and overheads. Furthermore, the two DCA strategies are compared to provide a guideline on how to trade off between the channel reuse efficiency and the control overhead.