Abstract Optical and spectroscopic properties of Langmuir–Blodgett (LB) films of N, N-dioctadecyl- p-nitrophenylaniline (DONPA) mixed with cadmium arachidate (CdA) were measured to characterize the orientation of molecular and structural effect on the second harmonic generation (SHG) intensity of the films. The LB films were fabricated with various molar ratios of DONPA and CdA to investigate the orientation effect of DONPA on CdA in the mixed LB films. The molecular orientations of molecules in LB films were examined using the polarized transmission and reflection–absorption FT-IR and the UV-visible absorption technique. The SHG intensities of LB films were measured. Chromophores of DONPA formed the J-aggregates. DONPA (in a 1:1 mixture of DONPA:CdA) LB film was oriented with a polar angle of 61±3° from the surface normal, and has azimuthal orientation which is parallel to the dipping direction.