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The gas chromatography-mass spectrometry (GC-MS) multi-residue method with different ionization modes has been developed for the determination of twenty-three organochlorine pesticides (OCPs) and ten polychlorinated biphenyls (PCBs) in crayfish.The approach was based on an extraction with acetonitrile saturated by hexane (containing 0.1％ acetic acid),followed by a dispersed-solid phase extraction (SPE) cleanup with primary secondary amine (PSA) and octadecylsilane (C18) and finally analyzed by GC-triple quadrupole MS operated in electron impact (EI) and positive chemical ionization (PCI),and GC-MS in negative chemical ionization (NCI),respectively.An isotope internal standard,DDT-D8,was used for quantification.These three GC-MS procedures were validated and compared using crayfish samples fortified at three levels (5,10 and 20 ng/g).Good linearity,accuracy,precision and sensitivity were achieved by all three ionization modes with recoveries in the range of 70％ to 112％,relative standard deviations (RSDs) below 10％ and limits of detection (LODs) in the range of 0.002 to 8 ng/g.Extremely high selectivity was obtained by both EI- and PCI-MS/MS,while PCI was more sensitive to compounds containing nitro and carbonyl groups,and EI was a better choice for dieldrin,endosulfan and its metabolites.NCI-MS had the highest sensitivity but lower anti-interference ability,and was not suitable for the determination of DDTs and PCBs.These three ionization modes can be used as complementary and alternative in routine GC-MS analysis of organochlorine pesticides and polychlorinated biphenyls in food.