Although a number of asynchronous physical-layer network coding (PNC) schemes have been proposed, synchronous PNC may still be preferred for its simplicity. In this paper we propose a new symbol-level synchronization scheme which reduces the misalignment to less than one hardware clock cycle period. In addition, we prototype the proposed scheme on the Universal Software Radio Peripheral (USRP) platform and demonstrate that USRP keeps synchronization for a considerably long time. Our experimental results show that the synchronization improves the goodput of two-way relay networks with PNC and allows users to communicate using higher symbol rates.