Organic light-emitting devices (OLEDs) with emission peak at 520 nm were designed. The electroluminescence (EL) spectra including the integrated intensity, the peak width at half height, and the intensity and the position of the peak of the EL spectra of the OLEDs and microcavity OLEDs (MOLEDs), the total thickness of organic layers which is changeable, were calculated and theoretically analyzed with the thickness of the layer of NPB and light-emitting layer of Alq3 ranging from 10 to 100 nm, respectively. According to these studies, it was found that the optimized OLEDs should be constructed with 70 nm NPB and 62 nm Alq3, and this structure should be more suitable to configurate the MOLEDs. These results suggest that the suitable structure of OLEDs/MOLEDs could be designed with help of theoretical calculation, which is also helpful to the light-emitting properties of OLEDs and MOLEDs.