The preliminary antifertility test of Dictamnus albus(Rutaceae) showed that a methanol extract of root bark decreased fertility in rats when administered orally on days $1{sim}10$ postcoitum. Hexane, chloroform, butanol and aqueous fractions from the methanol extract, were tested to produce the result that only the hexane fraction exhibited positive activity. The hexane fraction was subfractionated into acidic, alkaloidal and neutral subtractions and tested. Neutral subtraction showed positive activity. From the neutral subtraction, fraxinellone was isolated as an active principle. The timed-dosing experiments revealed that antifertility activity of fraxinellone appeared to be exerted after tubal exit of the ova to prevent implantations.