Molecular aggregation and orientation in mixed Langmuir-Blodgett (LB) films of 5-(4-N-octadecylpyridyl)-10,15,20-tri-p-tolylporphyrin (porphyrin 338a) and stearic acid have been investigated by means of ultraviolet-visible (UV-vis), fluorescence, and infrared (IR) spectroscopies. It has been found that for the multilayer mixed LB films deposited onto glass and gold-evaporated glass substrates, the formation of porphyrin aggregates is facilitated by adding stearic acid, evidenced by a 3-6 nm red shift of the Soret band in the W-vis spectra and a 5 nm red shift of the Q(x)(0,0) band in the fluorescence emission spectra, compared to the corresponding LB films of porphyrin 338a. This result and that for the one-layer mixed films suggest that fatty acid molecules impose their effect on promoting the porphyrin aggregation in the mixed films via interlayer interactions. Comparison of IR spectra between the mixed films and those of porphyrin 338a leads to the conclusion that the alkyl chains of porphyrin 338a molecules in the mixed LB films become ordered and closely packed and more perpendicular to the substrate surface compared to those in the LB films of porphyrin 338a. The comparison of ratio of intensities of CH2 antisymmetric and C=N (pyridyl group) stretching bands in the IR spectra between mixed LB films and the LB films of porphyrin 338a also supports this conclusion.