To clarify the effects of cathodic protection and seawater on cavitation erosion, charging the cathodic current of 0-1 A on specimen of Ti-6Al-4V alloy was carried out using a cavitating jet erosion apparatus in tap water and seawater. The results showed the following : (1)Erosion resistance of Ti- 6Al-4V alloy in seawater is excellent, as it is in tap water, but the volume loss rate is slightly influenced by seawater. (2)The volume loss rate of Ti-6Al-4V alloy is remarkably affected by the cathodic current. (3)The volume loss rate of Ti-6Al-4V decreases by about 0.7 times in tap water and 0.6 times in seawater compared to that of stainless steel SUS316. (4)Under cathodic protection, the cushioning effect of the protective film of the adhesion product reduces the volume loss, as do the hydrogen gas effect and the anticorrosive effect.