Structure and electrical conductivities of the mixed LB films of dioctadecyldimethylammonium-Au(dinit)(2) (2C18-Au) and arachidic acid (C20) are studied with various mixing ratios. The in-plane conductivity at room temperature decreases with increasing the molar mixing ratio of C20. Both the as-deposited and electrochemically-oxidized LB films give sharp X-ray diffraction peaks at 2 theta = 3.15 degrees indicating a d-spacing of 2.8 nm which is identical to that of pure 2C18-Au system. It is suggested that the 2C18-Au complex forms an identical layered structure both in the pure and the mixed systems.