The aim of this study was to reduce the loss of preliminary treatment of sample such as derivatives and emulsification, to enhance the maneuverability and accuracy of method, to promote the in stability of detection. Residues of pentachlorophenol and its sodium salt in edible tissues of crucian, freshwater shrimp, cod and chinese mitten crab were determined by gas chromatography using internal standard method. Such factors as derivation method, Whether n-hexane was blowed or not for pentachlorophenol and its sodium salt were compared in this paper. Moreover, the chromatographic conditions were surveyed and optimized for detection of the residues of pentachlorophenol and its sodium salt in edible tissues. The target compounds were first extracted two times with n-hexane in an acidic medium, and then back extracted with potassium carbonate solution. The extract was derived with acetic anhydride, and the derivative was extracted with n-hexane. The HP-5MS (30 m×0.32 mm×0.25 μm) column and the micro-cell electron capture detector was used for the determination. This method chooses 2,4,6-TBP as internal standard substance. The working curves were linear (R2=0.9996) in the range of 0.2-20.0 μg/L. When spiked with 2.0, 5.0, 10.0, 50.0 μg/kg of pentachlorophenol and 50 μg/kg of 2,4,6-tribromophenol, the average recoveries were 85.8%-109.0% and the RSD were 0.8%-9.3%. The detection limit was 0.6 μg/kg.