Thin films of RuO2 were prepared by reactive rf sputtering from a Ru target in an Ar + O-2 atmosphere, with the oxygen concentration varying from 10% to 100%. The resistivity, deposition rate, crystal structure: surface morphology, composition and chemical binding state of RuO2 films were investigated. RuO2 films sputtered with 33% or more oxygen exhibited lower deposition rate and lower resistivity relative to the films sputtered with 10 or 20% oxygen. The O/Ru ratios of the RuO2 films sputtered with 10, 20 and 50% oxygen were similar; however, evident rutile structure was only observed for films sputtered with 50% O-2. The low resistivity is related with the crystallinity of the films. The RuO2 films sputtered with 50% oxygen were annealed in flowing oxygen at 200-750 degreesC for 30 min. The film resistivity was further decreased with increasing annealing temperature and reached a minimum value of 110 mu Ohm cm after annealing at 650 degreesC. From X-ray diffraction and scanning electron microscopy analyses, the decrease of film resistivity upon annealing is attributed to the grain growth and reduction of lattice spacing in the RuO2 films.