Fly ash from coal-burning plants is used in concretepromarily because of its pozzolanic and cementitious properties. The classified fly ash, studied in this research, is produced by air classification of its conventional fly ash to the classes having the maximum particle diameter of 20 microns. The main objects of this study is to detail effects of the classified fly ash addition in concrete in producing high quality and highly durable concrete under marine environments. This report gives an experimental results on the durability of concrete against sea-water attack and chloride attack under marine environments.