The concentration of 137Cs in surface water of the East China sea and adjacent region (128°00'-129°00′E, 26°28′-32°33'N) was determined in 1978. The values vary from 0.047 pCi/L to 0.244 pCi/L. The mean value is 0.140 pCi/L.It shows that radioactive contamination of Caesium 137 is low. Based on the data obtained by us and other workers, we plotted its yearly change against its concentration as shown in Fig. 1.