Infrared optical properties and the relevant structure of a new series of nanoparticulate alpha-Fe2O3-stearate alternating Langmuir-Blodgett films (FeNSF) were studied by FTIR absorption, horizontal grazing reflection absorption spectra and Kramers-Kronig analysis. The size-dependent band progressions relevant to the trans-zigzag planar structure of hydrocarbon chains in FeNSF were detected. The size effect of two asymmetric stretching vibration bands of the carboxylate group was determined. The shifting of interface COO- bands is considered as a quantum size effect of vibration energy level. The order structure and size effect of the films were studied by the linear infrared dichroism of the films and the molecular orientation calculation results.