Abstract: Carbon-14 labelled chlorpyrifos, (1,O,O-diethyl O-(3,5,6-trichloro-2-2,6-¹⁴Cpyridin-2-yl)phosphorothioate) was synthesized from barium-14Ccarbonate via nine-step radioactive reaction including Grignard reaction, nucleophilic substitution, intramolecular cyclization, dehydration aromatization, electrophilic chlorination, etc. The labelled product (30.2 mCi) was obtained after RP-HPLC purification in the overall radiochemical yield of 11%. The product was characterized by ESI-MS, ¹H NMR and HPLC-FSA. Its technical data (radiochemical purity>98%, chemical purity>98%, specific activity 55.2 mCi/mmol) was determined by TLC-IIA, HPLC-LSC, HPLC-FSA/PDA/MS, and LSC. This carbon-14 labelled chlorpyrifos can be used as radiotracer in the study on its pharmacokinetics, eco-environmental security, mechanism of pollution reduction, and so forth.