Cavitation erosion was studied for copper-based alloy samples (brass, high-power brass, bronze, phosphor bronze and aluminum bronze) using a vibratory apparatus in seawater and distilled water. Similar experiments for those copper-based alloy samples in seawater clarified the effects of cathodic protection on cavitation erosion. The volume loss of copper-based alloy in seawater is less than that in distilled water when the corrosion films is formed : the impact load by the bubble collapse pressure is reduced by the films' cushioning effect. Under cathodic protection, cushioning effect of the protective film of the adhesion product decreases the volume loss, as do the hydrogen gas effect and anticorrosive effect, etc. Cavitation erosion resistance of the copper-based alloy in seawater and under cathodic protection can be evaluated as well as in distilled water in macro hardness (HV). These are improved when the average hardness increases.