A plasma-solid surface reaction on the nanometer scale was observed in situ for the first time. This was achieved by employing special scanning tunneling microscopy (STM) that could be operated in a plasma environment. To prevent current due to noise from the plasma, the probe tip was coated with epoxy grease and insulating glass. The apex of the probe tip was sharpened by a focused ion beam (FIB). This special STM had sufficient stability and reproducibility to measure a 10 x 10 mum(2) region for more than 5 h in a plasma environment. Using this apparatus, sequential in situ STM images of highly oriented pyrolytic graphite (HOPG) were obtained in a radio-frequency (rf) (430 MHz) low-pressure (1 torr) air glow plasma. These images revealed that a projection of over 100 nm in height was etched by the plasma and removed.