A new diterpenoid quinone, agastaquinone [1], was isolated from the roots of Agastache rugosa. An oxime derivative [2] of agastaquinone was prepared with hydroxylamine hydrochloride. The structure of agastaquinone [1] was established as 7-hydroxy-12-methoxy-20-norabieta-1,5(10),6,8,12-pentaene-3,11,14- trione by spectroscopic techniques. Compounds 1 and 2 showed nonspecific cytotoxic activities against several human cancer cell lines in vitro (A549, SK-OV-3, SK-MEL-2, XF498, and HCT15).