A high-pressure turbulent flow reactor coupled with a chemical ionization mass-spectrometer was used to determine the branching ratio of the HO2 + NO reaction: HO2 + NO OH + NO2 (1a), HO2 + NO HNO3 (1b). The branching ratio, = k1b/k1a, was derived from the measurements of "chemically amplified" concentrations of the NO2 and HNO3 products in the presence of O2 and CO. The pressure and temperature dependence of was determined in the pressure range of 72-600 Torr of N2 carrier gas between 323 and 223 K. At each pressure, the branching ratio was found to increase with the decrease of temperature, the increase becoming less pronounced with the increase of pressure. In the 298-223 K range, the data could be fitted by the expression: (T,P) = (530 ± 10)/T(K) + (6.4 ± 1.3) Å~ 10-4P(Torr) - (1.73 ± 0.07), giving 0.5% near the Earth's surface (298 K, 760 Torr) and 0.8% in the tropopause region (220 K, 200 Torr). The atmospheric implication of these results is briefly discussed.