Amorphous Si films prepared by rf sputtering have very different properties depending on the Ar gas pressure and the substrate-target spacing during sputtering. The electrical conductivity at room temperature is decreased by about five orders of magnitude by changing the Ar gas pressure from 2×10-2 to 3×10-1 Torr. Analyses by electron spin resonance and electron probe microanalyzer revealed that films prepared under low pressure contain more defects and Ar atoms. Since infrared absorption, presumably due to Si–O stretching vibration, appears in high-pressure samples, the effect of oxygen was also investigated by intentionally incorporating oxygen into the films during sputtering. These films, however, behaved defferently from the films sputtered under high pressure.