The thermal property of octadecyldimethylamine oxide (C(18)DAO) multilayer Langmuir-Blodgett (LB) film was investigated by infrared reflection-absorption (IRRA) spectrometry and two techniques of molecular orientation analysis. A conventional estimation theory of molecular orientation is expanded in the present study so that the first layer of the LB film that shows thermal properties significantly different from other layers is discussed separately. The results by the conventional analytical technique suggest that the multilayer LB film is thermally disordered at 50 degrees C and melts at 60 degrees C. The new technique, on the other hand, suggests that only the first layer of the LB film is greatly disordered between 50 and 60 degrees C, prior to the entire melting above 60 degrees C. The newly proposed technique gives insight into the structure of a LB film that is not uniform in structure that depends on thickness. The differing of thermal properties of the first and the rest of the layers revealed by the new method suggests that the headgroup of C(18)DAO has a uniquely strong interaction potential with,the gold surface, probably due to its strong dipole.