This paper describes both experimental and theoretical investigations on sloshing motion of liquid in cargo tank and resulting tank wall pressure. Motion of liquid is numerically simulated by the MAC method, where the effects of tank oscillation are expressed as apparent forces with a moving coordinate system in order to simplify the boundary conditions. The calculation successes to explain the suppression and the resonance shift of liquid motion caused by chamfers. The calculated time histories and amplitudes of sloshing pressure also show good agreement with experimental values. The present method estimates sloshing pressures more precisely than the linearized theory in design procedure of tank structures.