The delocalization of a-electrons from bonds to bonds in three and four-membered ring molecules was studied in a theoretical manner. The antibonding property of the a-electron delocalization between geminal C-C bonds increases in perfluorocyclopropane (5) and cyclopropene (8) while it decreases in cyclopropane (2). This accounts for the fact that the three-membered rings (5 and 8) are more strained than the four-membered rings (6 and 9) while the ring strains of 2 and cyclobutane (3) are comparable. The geminal delocalization between ring bonds is bonding in the three-membered ring molecules O-3 (14) and S-3 (23) and less antibonding in N3H3 (11) Ab initio molecular orbital calculations of the strain energies revealed that the three-membered rings (11, 14, and 23) should surprisingly be less strained than the four-membered rings (12, 15, and 24). The present findings contribute to establishing the geminal delocalization theory for ring strains.