"The reason why Chinese users could not log on to the website was that our domain name server (DNS) in the United States was illegally attacked," Xinhua reported.
The company said it was still investigating the problem.
"It is rare for Baidu.com to be down for so long as the company boasts high security protection," said Li Tiejun, an IT security engineer of Beijing Kingsoft.
The search engine, which claims 70 percent of China's Internet search market, had only been down only once previously, for half an hour in December 2006.