Abstract: This report summarizes the radioactive background measured in the coastal waters of South China Sea for three recent years. The sea area is located at 110°—117°E and 20°—24°N. The items included sea water, marine organisms and sediments. The radioactive materials for detection included: gross U, gross β,~(238)U,~(232)Th, ~(226)Ra,~(40)K, ~(90)Sr,~(137)Cs, ~3H, etc. The contents of radioactive materials in sea water are: gross U, (25.6—40.9)×10~(-3) Bq/1; ~(90)Sr, (2.5—4.7)×10~(-3) Bq/1; ~(137)Cs, (2.0—4.4)×10~(-3) Bq/1; gross β, (55.5—87.5)×10~(-3) Bq/1 and ~3H, 0.33-0.82 Bq/1. The contents of radioactive materials in the sediments are ~(238)U, 51.2—99.7 Bq/kg(dry); ~(232)Th, 38.1—62.1 Bq/kg(dry); ~(226)Ra, 24.3—59.1 Bq/kg(dry); ~(137)Cs, 1.5—2.4 Bq/kg(dry); ~(40)K, 416.3—876. 1 Bq/kg(dry) and gross β, 569.2—912.2 Bq/kg(dry). The contents of radioactive mateials in the marine organism are ~(238)U, 21.0—99.2 Bq/kg(dry); ~(232)Th, 30.4—111.5 Bq/kg (dry); ~(226)Ra, 7.1—34.7 Bq/kg(dry); ~(40)K, 1069.2—2132.7 Bq/kg(dry); gross β, 1020.0—2492. 0 Bq/kg(dry). The radioactive contents in fishes are: ~(238)U, 4.2—12.0 Bq/kg(dry); ~(232)Th, 4.7—57.9 Bq/kg(dry); ~(226)Ra, 1.3—7.1 Bq/kg (dry); ~(137)Cs, 0.4—0.9 Bq/kg(dry).