A continuous nitrate free radical (NO3) beam is produced in situ by the pyrolysis of pure gas phase N2O5 at 280 degrees C (+/-0.5 degrees C) in a double-heater inlet system on a double-chamber UPS Machine-II which was built specifically to detect transient species. The HeI photoelectron spectroscopy (PES) of the NO3 radical is recorded for the first time. Five obvious bands emerge on the PE spectrum of the NO3 radical. A very sharp peak with the lowest ionization energy at 12.55+/-0.01 eV originates from electron ionization of the HOMO for the NO3 radical, corresponding to the NO3+((1)A(1)’)<--NO3 ((2)A(2)’) transition. To assign the bands of the PE spectrum of the NO3 radical, the improved density functional theory (DFT) calculation based on the Amsterdam density functional (ADF) program package has been carried out according to D-3h symmetry for the neutral ground state of the NO3 radical and the equilibrium geometries of several ionic states of the cationic species. The ionization energies computed are in reasonable agreement with the PES experimental ionization energies. Both PES experimental and DFT calculation provide evidence on the lowest triplet and singlet states of the NO3+ cation.