Cao County or Caoxian (simplified Chinese: 曹县; traditional Chinese: 曹縣; pinyin: Cáo Xiàn), or Caoxian is a county of southwestern Shandong province, People's Republic of China, bordering Henan province to the west. It is under the administration of Heze City.
The population of the county was 1,394,034 in 1999.
Bo, one of the capitals of the Shang dynasty, lay in what is now Cao.
Prior to the Boxer rebellion, Cao was an area noted for its lawlessnes, and during the Qing dynasty, when local garrisons were weakened due to the First Sino-Japanese War, a militia known as the Dadaohui ("Great World Company") was set up, which operated until 1895, and may have inspired some of the Boxer rebellion.