Electron impact ionization of nitric oxide was studied as a function of electron energy up to 180 eV. A double focusing mass spectrometer in combination with an improved electron impact ion source (Stephan et al.) was used, alleviating the problems of ion extraction from the source and the transmission of the extracted ions through the mass spectrometer system. Relative partial ionization cross section functions were measured for the processes NO+e→NO++2e and NO+e→NO2++3e. An accurate determination of the cross section ratio q(NO2+/NO)/q(NO+/NO) was made. In addition, absolute partial ionization cross sections for the production of NO+ and NO2+ were determined by normalizing the relative cross sections with two different methods. Using nth root extrapolation an appearance potential of NO2+(X 2Σ+) was determined. The result is compared with previous measurements and theoretical calculations.