Thin silver island substrates with different sizes and different states of aggregation of the silver particles are prepared by evaporation and a thermal annealing process. The Langmuir-Blodgett monolayer of 4-octyl-4＇-(3-carboxytrimethyleneoxy)azobenzene are deposited on these silver-coated substrates, and their infrared spectra are studied. The experimental results reveal that the enhancement factor decreases as the silver particles become less aggregated.