A L-type anti-pitching tank which has a duct with an opening at one end is studied. Characteristics of the L-type anti-pitching tank are as follows. 1) Zero response frequency does not exist. 2) Anti-pitching moment, when the opening of a duct is turned forward, is about 30% larger than backward. 3) Smaller damping of tank water and larger anti-pitching moment can be obtained, as compared with ordinary anti-pitching tanks.