The morphologies of LB films of arachidic acid and its salt were observed by atomic force microscopy (AFM) to reveal their cation dependence from the viewpoint of ionic radius and solution pH. The salt films having divalent metal ions (Ba2+, Sr2+ and Co2+) except for Mn2+ showed similar characteristic stripe-shaped macroscopic structures. An area per molecule in the film increased with decrease of ionic radius. Concerning the solution pH, significant changes in molecular arrangements and macroscopic structures were observed in the barium arachidate films. Coexistence of 2×2 superstructures with 1×1 fundamental structures were observed at pH=8.0, but 3×1 at pH=9.8. These structures were considered to depend on the increase of the salt forming ratio increases with pH.