This paper describes the alga reef made from ferrite mixtures which are obtained by mixing ferrite grains with various binders-asphalt, cement, rubber, resin etc. Ferrite mixtures have been applied as a magnetic. markers system and mechanical vibration damper. Moreover, we have tried to develope new applications of ferrite mixtures in addition to current uses. One applicatiop into the field of aquaculture is the use as an alga reef. From the view point of practice, three types of alga reeves were considered. 1) Sprayed type Slurries of ferrite mixtures are sprayed the alga reef of which main structures are constructed by current materials. 2) Cemented type Blocks of ferrite mixtures are cemented to the alga reef of which main structures are constructed by current materials. 3) All ferrite type Main structures themselves of alga reef are constructed by ferrite mixtures. As ferrite mixturos are heavy, high strength and rapid hardening, we can make the stable and strong alga reef against the wave forces, and can save the curing cost. Moreover, we can find out the alga reef in the sea easily by using the magnetic markers system.