Capacitively coupled plasma reactive ion etching of silicon by injecting nitrogen trifluoride (NF3) gas was conducted, and the etching process was studied with a residual gas analyzer and an optical emission spectroscopy. During the etching of silicon by the NF3 plasma, the currents of F+ and SiF+3 ions were obtained from the residual gas analyzer. At the same time, the line intensity of fluorine was measured using optical emission spectroscopy. The ion currents of F+ and SiF+3 and the line intensity of fluorine were highest for etching at high pressure. With the results of this study, the general characteristics of reactive ion etching of silicon by using NF3 plasma were confirmed. In addition, by simply changing the flow rate of NF3, a high NF3 flow rate was found to result in a high etch rate because of the reduced residence time.