In this study, we investigated the potential changes of a rectangular cross section having different aspect ratio and area. A 2D Poisson-Boltzmann equation was used to model the electric double layer field of the cross section. The potential change of the cross section was studied numerically with FEMLAB 3.0 to understand the geometry effects of the rectangular microchannel used for electroosmotic flow. According to the result, the cross-section geometry shows significant influences on the potential field. The potential distribution shows same tendency with various cross-section areas in the same aspect ratio. The potential gradient increases with the increase of the cross section area.