Inhibition of acetylcholinesterase (AChE) activity by 17 kinds of bicyclic monoterpenoids wasinvestigated. Bicyclic monoterpenoids are contained in many kinds of essential oils. Inhibition of AChEwas measured according to the colorimetric method. 3.1.1 and 4.1.0 bicyclic hydrocarbons with allylicmethyl group showed strong inhibition. (+)- and (−)-α-pinene and (+)-3-carene were potent inhibitorsof AChE. 3.1.1 and 2.2.1 bicyclic alcohols and ketones showed weak inhibition. 3.1.1 and 4.1.0 bicyclichydrocarbons with allylic methyl group were found to be uncompetitive inhibitors. Keywords: Acetylcholinesterase; bicyclic monoterpenoids; inhibition of enzyme activity; uncompetitiveinhibitor