Some actual data of cathodic protection by long life Al anode system applied to marine steel structures (sheet piling wharf, steel pile, pier, dolphin, etc.) were analysed and compared with impressed current system, from the viewpoint of economy and maintenance practice.The conclusions are summarized as follows.1) In common submarine environments, Al anodes giving initial current density for steel of 0.1A/m2 in sea water, and 0.02A/m2 in mud, polarize steel structures to the potential of Ca. -900mV (vs. Ag-AgCl half cell) within short periods and are expected to maintain this condition for the whole life of the anodes.Anode current density in this steady state is supposed to be 0.2-0.3mA/cm2, and it is recommended to keep the anode current density larger than 0.1mA/cm2, since the current efficiency of anode is larger.2) Long life Al anode systems are capable of sufficient protection to marine steel structures for longer than 10 years, and cathodic protection using these anodes are compatible with the impressed current system on an economic point even in the case of large structures.3) Judging from the weight losses of test pieces, it was confirmed that, for steel structures fitted with Al anodes, the protective effects in tidal zone, even in about 50% submerged portion, were practically equal to those in fully submerged portion.