The structures and properties of mixed monolayers of a luminous rare earth complex Eu(TTA)(4)CTA with octadecylamine (ODA) on an Eu(TTA)(4)(-) aqueous solution subphase surface have been studied. The plateaus, appearing on the pi-A isotherms of Eu(TTA)(4)CTA, ODA and Eu(TTA)(4)CTA/ODA (1:1, 1:2, 1:4) monolayers at 14 mN/m, correspond to a structural change process of the monolayers. The head groups closely packed monolayers change to long tails closely packed monolayers during this process. The observation of transmission electron microscopy (TEM), electron diffraction (ED) and fluorescence microscopy (FM) indicate that crystals of the complex are formed at the plateau for Eu(TTA)(4)CTA, Eu(TTA)(4)CTA/ODA (1:1, 1:2) mixed monolayers, and no crystal of the complex is formed at the plateau for Eu(TTA)(4)CTA/ODA (1:4) and ODA monolayers. The UV-vis spectra and fluorescence spectra of mixed LB films show that the different structures greatly affect the film properties.