With three-color triple resonance excitation, rotational-state-resolved laser multiphonon ionization spectra of highly excited ng Rydberg states (n=11-67, upsilon=0 and 1) of NO have been observed. Determining the term values of the observed states, we have found that those values are in good agreement with the theoretical values calculated on the basis of the long range force model. Through detailed analysis of the spectral intensity distributions, we have proved, for the first time, the theoretical prediction that even in g Rydberg states there are efficient vibrational and rotational autoionization processes much faster than the radiative decay. In addition, we have also found the presence of non-negligible predissociation processes competing with the autoionization processes, roughly estimating both predissociation and autoionization rates.