The phase-out of increasing numbers of synthetic nematicides has prompted an urgent need for environmentally benign and effective products to protect crops from nematode damage. We tested a group of selected plant-origin monoterpenoid compounds for their nematicidal activities, and found some of them could be potent nematicides. Most of the monoterpenoids were more toxic than the commercial nematicide oxamyl (13.4% mortality at 250 μg/mL) against the soil saprophytic nematodeCaenorhabditis elegans, among which carvacrol and thymol were the most potent (100% mortality at 250 μg/mL). (–)-Perillaldehyde, eugenol, geraniol and menthol were of moderate nematicidal activity at the same concentration, with mortality at 97%, 91%, 90% and 84%, respectively, againstC. elegans.Nematicidal activity against the root lesion nematodesPratylenchus penetranswas generally less potent, but citronellol, carvacrol and (–)-perillaldehyde were more toxic (mortality at 250 μg/mL were 87%, 78% and 56%, respectively) than oxamyl (50%). Thymol showed similar (50%) mortality to that of oxamyl.