A method for determination of malathion in natural water by cathodic stripping voltammetry using mercury film electrode is suggested in this paper. The method is based on the quantitativeformation of group generated by malathion as a result of elimination reaction inthe ethanol solution of sodium hydroxide. The group is similar to sulphide in electrochemical properties, and can be determined by cathodic stripping voltammetry. Interference from inorganic sulphide is prevented by the addition of Hg (NO3)2. This method has been applied to the determination of residual malathion in natural water (e. g. the sea, rivers, lakes, etc.). The waveheight is directly proportional to the malathion concentration within the range 5×10-9 - 1×10-7 mol/L. The method is simple and rapid, and the results of water sample determination is quite in accord with those of Gas Chromatograph. A method for determination of malathion in natural water by cathodic stripping voltammetry using mercury film electrode is suggested in this paper. The method is based on the quantitative formation of group generated by malathion as a result of elimination reaction inthe ethanol solution of sodium hydroxide. The group is similar to sulphide in electrochemical properties, and can be determined by cathodic stripping voltammetry. Interference from inorganic sulphide is prevented by the addition of Hg (NO3) 2. This method has been applied to the determination of residual malathion in natural water (eg the sea, rivers, lakes, etc.). The waveheight is directly proportional to the maththion concentration within the range 5 × 10-9 - 1 × 10-7 mol / L. The method is simple and rapid, and the results of water sample determination is quite in accord with those of Gas Chromatograph.