The research of this paper is on the free sloshing mode of liquid tank with baffles. The liquid tanks FEM model is made in ANSYS, and some investigation is done on its sloshing mode and the shape of free surface, under different filling ratio conditions. After comparing the tanks without baffles with the one with baffles, it is found that the sloshing mode, basic frequencies and free surface shape are all affected by baffles. And the same free surface shape appears in different liquid tanks, but in different mode. It also can be concluded that there are close mode frequencies in symmetrical tanks.