Remnants of the bridge's piers were unearthed in a suburb of Xi'an, the province's capital, China's state-run Xinhua News Agency reported.
The bridge is estimated to have been almost 1,000 feet long and 65 feet wide, Li Yufang, a researcher at the China Academy of Social Sciences, said.
The bridge may date to the Qin Dynasty of 221-206 B.C., Li said, and spanned the Wei River, the largest tributary of the Yellow River.
It probably linked two imperial palaces in the ancient capital of Xianyang, he said.
"It's the first time a wooden bridge of this size and age has been found, and it is of great value to the study of ancient bridge architecture and transportation," Li said.
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