The calculation models of the external quantum efficiency for the planar surface emitting devices are presented with considering their spatial distribution of radiance/luminance and relative spectral power distribution. These models can be applied to all kinds of planar surface emitting devices including OLEDs, VCSEL, MOLED, MCLED and planar THz emitting devices. Two planar emitting devices with and without cavity are fabricated with the structure of Glass /DBR /ITO /NPB /Alq ∶C545T /Alq /LiF /Al and Glass /ITO /NPB /Alq ∶C545T /Alq /LiF /Al. The external quantum efficiencies are calculated at different current densities using the corresponding model. The external quantum efficiency of the cavity device is close to 1.5% at a current density of 14 A/m2 and brightness of 100 cd/m2, and reaches 2% around 0.14 A/m2 and 2.2 cd/m2.