Clock synchronization is very important for power management protocol in a multi-hop MANET. However, since MANET is a network temporarily formed by a collection of mobile nodes without the aid of any centralized coordinator, clock synchronization is very difficult to achieve. Therefore, most of previous works on power efficiency assumed asynchronous clock. As a consequence, a mobile node will waste a lot of power and time waiting for forwarding a packet to its neighbors, due to the lack of information of wakeup times of its neighbors. In this paper, we propose a multi-hop time synchronization protocol, referred to as MTSP, for multi-hop MANETs based on IEEE 802.11 ad hoc mode. The MTSP consists of two phases: beacon window (BW) phase and synchronization (SYN) phase. In BW phase, several devices, which can directly communicate with each others, form a synchronization group. And each group selects the device with fastest timer as the leader node of the group. In SYN phase, leader nodes then synchronize with each other. Our simulation results show that MTSP is a distributed and effective multi-hop time synchronization protocol, especially for dense networks.