Magnetic Fe3O4-C18 composite nanoparticles of approximately 5–10 nm in size were synthesized and characterized by IR spectroscopy, atomic absorption spectroscopy, X-ray diffraction, and transmission electron microscopy. The magnetic Fe3O4-C18 composite nanoparticles were applied for cleanup and enrichment of organophosphorous pesticides. Comparative studies were carried out between magnetic Fe3O4-C18 composite nanoparticles and common C18 materials. Residues of organophosphorous pesticides were determined by gas chromatography in combination with a nitrogen/phosphorus detector. The cleanup and enrichment properties of magnetic Fe3O4-C18 composite nanoparticles are comparable with those of common C18 materials for enrichment of organophosphorous pesticides, but the cleanup and enrichment are faster and easier to perform.