Abstract: An Isotope Dilution mass spectrometry (IDMS) method for determination of 20 polycychlorinated Biphenyls (PCBs) in cod fish has been established. 20 PCBs were extracted by Aceton/Hexan (1/1) soxhlet extraction under 70 ℃. An gel permeation chromatography (GPC) and solid phase extraction (SPE) were used for cleaning up the extract. 20 PCBs were quantified by HRMS-MID, single point method. The recovery was in the rang 92%-105%. The detection limit of 20 PCBs was 8.9-9.4 ng•g^－1. The RSD (n＝6) ranged from 2.9%-4.6%.