The bond dissociation energy of the C-H bond at the a-position of an ether oxygen in cyclopentyl methyl ether (CPME) has been evaluated theoretically to be 393.3 kJ/mol by using an isodesmic reaction method. Essential factors accounting for this rather higher bond dissociation energy in CPME comparable to secondary C-H bond have been investigated on the basis of the structures and their thermodynamic data such as heats of formation (Delta H-f degrees) of CPME radical and relevant ether radicals evaluated by the theoretical approach.