Chloride attack is the primary cause of corrosion problem of concrete structures operate in marine environment. Therefore, concrete structures operate in such environment cannot escape from this corrosion related problem. This paper describes assessment of a reinforced concrete canal structure of power plant after 20 years of exposure in a marine environment. The work covers visual inspection of the structure, on-site and laboratory tests of the structure, analyses the current structural strength based on the tests, and proposing repair and/or strengthening for weak elements. Strength prediction is carried out using average and worst case scenarios. The strength calculations assuming average case scenario shows that by 2025 all the canal have no strength reduction due to corrosion. Calculation using the worst case scenario shows that all the canal by 2025 still comply with the limits specified in Indonesian Concrete Code, even though their strength has been reduced due to corrosion of the reinforcement.