Time synchronization is very important to coordinate data acquisition in wireless sensor networks (WSN's). In this paper, we propose a physical-layer time synchronization scheme using ultra-wideband (UWB) signals. The synchronization is performed by emitting reference UWB signals at pairs of neighboring nodes alternatively. Once one node receive the UWB signal from the other of the pair, the local time instant (LTI) information of the arrival of the UWB signals can be obtained and recorded in the processing units of the pair. According to these LTI information, the clock frequency drift (CFD) of a given (beacon) node can be estimated by the gradient descent algorithm. The time offset difference between the target node and the beacon node can also be estimated after the CFD of the target node is corrected. Simulation results of a ten-node network synchronization show the effectiveness of the proposed scheme.