Ruthenium dioxide thin films with an average grain size of about 12-50 nm were deposited by the reactive sputtering method. In this study, we found that the factors correlated to the resistivity of the films also govern the average grain size of the films. The grain-boundary scattering model fits well for the films, with a grain-boundary reflection coefficient R of about 0.785. Such a high value shows that grain boundary scattering is an important electron scattering mechanism for the RuO2 thin film. From the high value of R, it may be deduced that the resistivity of the film can be ten times higher than that of the bulk material and the film will still behave as a metal.