Thin polymer films need to be stable on solid substrates while processing application to preserve their properties. Increasing temperature causes polymer thin films lost their stability. From classical knowledge, the existence of impurity in polymer thin film usually induces dewetting behavior. However, addition of small amount of nano-fillers enhanced thermal stability of polymer thin film. In this research, thermal stability of composite film was investigated by adding zinc oxide nanoparticle into polystyrene thin film. The concentrations of zinc oxide nanoparticles were 0, 0.1, 0.2, 0.5 and 1.0 wt% in polystyrene (MW = 35,000 g/mol). After annealing the composite films at 90 °C for various times, the film morphology was studied by using optical microscope. Contact angle and surface energy of films were calculated by using contact angle measurement. Dewetting area was observed and considered to compare the percentages of each samples. From these results, we found that addition of zinc oxide nanoparticles can retard dewetting behavior of polystyrene film and increasing amount of nanoparticles increases thermal stability of composite films. Keywords: dewetting, nanoparticles, thin film IntroductionPolymer thin film is a famous material used in various technological applications such as dielectric coatings, optical coatings, lithography resisting, electronic packaging, and surfaces lubricating because of a remarkable insulation property of polymer. The thin film need to be stable on the substrate in technology application. However, because of the difference of chemical structure between polymer film and metal oxide substrate, polymer film trend to separate from the substrate which is called dewetting behavior. The first step of dewetting process, early stage, occurs when small holes are created. Subsequently, the small holes are expanded and merged which each others showing ribbon structure, intermediated state. Finally, ribbon structure is disconnected and the film presents a number of droplets which is called completely dewet.Many researches have been investigated how to retard dewetting action of polymer thin films, for example, modification of polymeric structure and/or substrate surface (Henn et al