Isotope dilution mass spectrometry (IDMS) and ultra-clean room approach are successfully used in the determination of Cd, Pb and Zn in seawater and the sample pretreatment of analysis.The precision in the determination of Cd, Pb and Zn in seawater is better than ±3% and the limits of detection are 6×10-12, 6×10-11 and 2.4×10-10 respectively.The effects of sample pretreatment such as filtration and acidification on the results of determination of Cd-Pb and Zn in seawater are disscussed.