The electron impact ionization of Ne, Ar, Kr, and Xe is considered for the impact energies varying from the threshold of ionization to 20 KeV. The method of Khare and Padalia (1970), modified to include approximately the effect of multi-ionization, has been employed. It is shown that the experimental data of different investigators differ from one another by about 20 to 30% (Kieffer and Dunn, 1966). It is noted that for neon the cross section values obtained by Schram et al. (1965, 1966) are smaller than those obtained by Rapp and Englander-Golden (1965), whereas the reverse is true for xenon. The agreement between theory and experiment may be regarded as satisfactory over the whole energy range.