Abstract: Ag/Ag2S crystal membrane electrode was prepared by chemical deposition of a silven layer on the glass capillary and then a layer of Ag2S was deposited electrolytically onto the silver layer. An electrode system was assembled between the Ag/Ag2S membrane electrode outside the capillary and the Ag/AgCl reference electrode placed inside the capillary. It was shown by the results of electrochemical study of the composited electrode system,that it was used successfully in the determination of CN--ion in sewage. Linear Nernst response was obtained in range of 1.00×10-3-1.00×10-7mol·L-1 of CN--ion or of pCN- 3-7,with a detection limit of 5×10-8mol·L-1,and the Nernst slope was found to be 110 mV/pCN-. Analytical results obtained by using this electrode in the analysis of 4 different sewage samples,checked quite well with those obtained by the standard photometric method,giving relative error less than 5.1%. Interferences and methods of elimination of some commonly co-existing ions (e.g. S2-,Ag+,Hg2+ etc) were tested. Mechanism of the electrode reaction was also discussed briefly.