In the design of super tankers or LNG carriers, which transport a large amount of liquid in the cargo tanks, the structural d11mage due to liquid sloshing is an important problem. The impact pressure from sloshing is most violent when the liquid motion of a partially filled tank is in resonance with the motion of a ship. In this study, the sloshing natural periods of a baffled tank, often installed to reduce liquid motion, is analyzed. A variational method is adopted to estimate the sloshing natural periods for a prismatic cargo tank with baffles of arbitrary filling depth of liquid; the results are compared with Lloyd's Register regulations on sloshing periods. In this study, using an effective liquid-fill-depth concept, sloshing periods for a baffled tank can be expressed by the same form as rectangular prismatic tanks without baffles. In contrast to Lloyd's Register regulations, which can be applicable only to cargo tanks with constant baffle size and distribution, the present results can be applicable to cases of variable baffle size and distribution.