The optical-optical double-resonant multiphoton ionization(OODR-MPI) technique has been applied to the study of the Rydberg states of nitrogen dioxide. The results show that ,althougy the OODR-MPI spectra of NO2 are composed of regular progression bands at different pump laser intensities, their ionization pathways are different.The NO2 mollecule is ionized through the (3+1+1)double-resonant process as the pump laser intensity is in a high value, or else it is through the (1+2+1)rpocess.The final resonant states in the two ionizing processes have been attributed to different Rydberg states.