Composite films consisting of metallic Cu nanoparticles dispersed in poly(acrylic acid) (PAA) have been prepared by reduction of Cu2+ from the copper salt of PAA above 220 °C under a H2 atmosphere. Optical absorption properties and structures have also been investigated by UV/VIS, WAXD, TEM and IR. Spherical Cu particles were found to be homogeneously dispersed in the PAA and the diameters of the particles were in the range 10–16 nm. The composite films exhibited an optical absorption peak centered at ca. 570 nm, which was attributed to surface plasmon resonance of Cu nanoparticles. The composite film made by heat treatment at 220 °C was less stable because Cu particles in the film were oxidized to Cu2+ ions within several weeks, while the composite films prepared by heating above 230 °C were stable and the Cu particles in their films were not oxidized. The stability of the Cu nanoparticles in PAA is suggested to be related to the formation of ketone groups by condensation reactions between carboxylic acids of PAA above 230 °C.