Ab initio calculations of 12-crown-4, 15-crown-5, and 18-crown-6 and their protonated species have been performed. As a reference molecule, l-(2’-methoxy)ethoxy-2-methoxyethane (glyme-2) is also examined. Three protonated crown ethers are found to have quite similar moieties to that of glyme-2H(+). The ring strain due to formation of intramolecular hydrogen bonds is a measure of proton affinity (PA) values. The present computational result supports Meot-Ner’s and Kebarle’s PA values of 12-crown-4. Those of 15-crown-5 and 18-crown-6 reported by Kebarle were very close to the present theoretical values.