This article presents an external modulation diode laser (with wavelength 661.85 nm and line width 0.3 nm) cavity ring-down spectrometer for measurement of NO3 radical in the atmosphere. The output spectrum of the diode laser is optimized by changing the external modulation signal; the effective cross-section of the instrument is a convolution of the measured cross-section and the laser spectrum. Interference of other gases (O3, NO2, and water vapor)in NO3 radical detection has also been investigated. Considering the loss from the PFA tube wall collision and the membrane filtration, NO3 radical inlet transmission efficiencies of the system is about 70% through the preliminary quantitative analysis. When the time resolution is 7 s, the laboratory detection limit of the system is 2.0 pptv. This instrument was deployed in the night atmosphere to measure the NO3 radical. NO3 concentration varied from 17.9 to 51.7 pptv with an average level of 36.3 pptv. A typical detection sensitivity in the night atmosphere is 3.5 pptv. Because of the uncertainty in the inlet transmission efficiencies and other factors, the total uncertainty of the measured NO3 radicals is about 8%(1 ). Experimental results show that the diode laser cavity ring-down spectroscopy can achieve high sensitivity in insitu detection of NO3 radicals in the atmosphere.