The corrosion rate of tank depends mainly upon the service condition of the tank, but the relation between the corrosion and the condition of the tank have not been unclarified in delails. We carried out several corrosion test of tank compartment in their different conditions. The corrosion rate of test pieces installed in tanks which were alternately filled with crude oil and kept empty (not filled with sea water) was 0.0720.23 mm/yr. and was severer than that have expected. In the case of tanks which was alternately filled with crude oil and dirty ballast sea water, the corrosion rate was slightly severer than that of the empty tanks. The tanks filled with clean ballast sea water was corroded as much as 0.240.36 mm/yr. and these values were several folds of the corrosion in the empty tanks.