The shutdown of unirule.org.cn comes amid a tightening of communication controls during one of China's biggest political scandals in decades.
"I have not been given a reason," said Mao Yushi, an 83-year-old economist and co-founder of the Unirule Institute of Economics in Beijing. "There was no prior sign. That's typical in China."
Unirule web administrator Zhang Liang said it had been down since Saturday but only temporarily for maintenance to solve technical problems. Zhang expected the site to resume Thursday.
Chinese authorities last month also suspended several leftist sites, including the ultra-leftist site Utopia, that had supported disgraced politician Bo Xilai and argued against privatization of the economy and other Western-style reforms. Utopia was ordered closed for one month.
Bo, the former party chief of Chongqing in southwest China, was removed from office and accused of "severe disciplinary violations." Bo's wife has also been named as the suspect in the murder of a British businessman.
Established in 1993, Unirule champions free market economics, and some of its members advise Beijing on its economic policies. Articles posted on Unirule's website focus primarily on economics and politics, Mao said. The think tank has not been affected, Mao said.
Mao has been named the 2012 winner of the Washington-based Cato Institute's Milton Friedman Prize for his advocacy for free markets.
He is scheduled to be in Washington on Friday to receive the award. He said from his Beijing home Wednesday that the blocking of the Unirule site had nothing to do with winning the Cato award.