The dispersion process of Ag nanoparticles into vapor-deposited nylon 11 thin films caused by heat treatment has been investigated. In situ optical transmission and Fourier transform infrared (FT-IR) reflection absorption spectroscopy were used independently for characterizing the changes in the surface plasmon resonance response of Ag nanoparticles and in the thermal behavior of the nylon 11 matrix during heat treatment, respectively. The peak wavelength of the plasmon band was observed to shift to shorter wavelength in the temperature range 40-80 degreesC. The infrared temperature study revealed that the as-deposited nylon 11 matrix is thermodynamically metastable and semicrystalline, including hydrogen-bonded small crystallites. These relaxed upon heat treatment above 40 degreesC, at which the Ag nanoparticles penetrated from the surface into the bulk phase of the matrix; These results demonstrate that there is a strong correlation between the optical spectral features, dispersion state of the particles, and structural change of the polymer matrix. Dispersion mechanism is discussed in terms of the surface free energy of Ag nanoparticles, which is reduced upon embedding in the polymer matrix.