The calculation of the external quantum efficiency(ηext) of organic light-emitting devices(OLEDs) is presented theoretically. The basic idea is to obtain ηext by calculating the fraction of the generated light that can escape through the top surface and the average transmittance over the escape cone. During the calculation, the effects of the interference, absorption and multiple reflections are neglected. Then a result of a typical 8-tris-hydroxyquinoline aluminum(Alq3)-based OLEDs on a planar glass substrate is obtained. And a twice increase in ηext can be achieved by using a high-index-glass substrate with an antireflection coating(SiO2) on substrate surface.