Oxidation and grain growth characteristics of copper thin films in an oxygen ambient were investigated. In general, reflow characteristics of thin films mainly depend on the surface self-diffusion of the film's material, but we found that the oxidation and grain growth could result in the reflow of copper thin films in an oxidizing atmosphere. Upon post-annealing at higher than 400 degrees C in an oxygen ambient, the normal grain growth and the agglomeration of copper occurred simultaneously and filled the patterns. This behavior was not observed in inert ambient annealings. Copper oxide was formed during the annealing in an oxygen ambient to a thickness of at most 20 nm. The grain growth is attributed to stresses produced during the surface oxidation of the copper.