The absorption cross section of NO3 free radical has been measured in the wavelength region of 652.5-672.5 nm at three temperatures (220, 240, and 298 K). A discharge flow apparatus was used to prepare NO3 via the F + HNO3→HF + NO3 reaction, and long-path continuous wave laser absorption cell, coupled to the flow tube, was used to measure the absorbance. The absolute concentration of NO3 in the absorption cell was determined by titration with NO. It was found that the absorption cross section at 662 nm (the peak of the band) increased with decrease in temperature, while the shape of the band itself did not change. These results are compared with those from other studies. The implication of this finding to NO3 measurement using absorption techniques is discussed.