The spreading behavior of poly (styrene-co-maleic anhydride)/arachidic acid mixed monolayers at the air-water interface was studied for determining the optimum preparation conditions of the Langmuir-Blod-gelt(LB) films. The stable mixed monolayers were able to be transferred successively onto solid substrates. The surface wettability of the built-up alms showed alternately the hydrophilic and hydrophobic property. Depending on even and odd number of layers. The structure of the mixed LB films was investigated by the X-ray diffraction measurements; the diffraction intensity decreased by the increase of poly(styrene-co-maleic anhydride) content, but the diffraction angles were not changed. The X-ray diffraction results also indicated that the mixed LB films have well-ordered layer structure.