Langmuir films of allophycocyanin (APC) formed at the air-water interface and transferred onto solid supports were studied by means of surface pressure-area (pi-A) isotherms and ellipsometry. The occupied area per molecule extrapolating the linear part of the pi-A Curve is identical with that when APC molecules were located at the air-water interface by disk plane parallel to the air-water interface. The thickness of the protein monolayer transferred on the substrate was measured by ellipsometry in order to determine the orientation of APC molecules in the air-water interface. Through observation of transmission electron micrograph of APC monolayer, we directly acquired the morphology about the orientation of APC molecules in the Langmuir-Blodgett (LB) films. Absorption spectrum of APC multilayers is different from that in the aqueous solution, but the fluorescence is the same as that in its aqueous solution. Comparative studies of circular dichroism spectra of the protein in aqueous solution and in LB film showed that conformational changes occurred, i.e., the alpha-helical APC molecule in aqueous solution is partially transformed into a beta-sheet in LB films.