The color purity of organic light-emitting devices is restricted mostly by the intrinsic character of the emitting material; however, the optical microcavity can improve the color purity by changing the structure of the devices. In this study, we demonstrated that a high color purity monochromatic top-emitting device can be obtained by changing the length of the microcavity. By adjusting the thicknesses of the hole transport layer and electron barrier layer, high color purity organic green phosphorescent top-emitting electroluminescent devices were fabricated. The structure of the devices was an Si substrate/Ag/ITO/NPB:F16CuPc (10 nm, 3%)/NPB (x nm)/TCTA (y nm)/mCP:Ir(ppy)3 (40 nm, 6%)/Bphen:Liq (30 nm, 40%)/Mg:Ag (12 nm, 10%)/Alq3 (35 nm). Direct green emission with chromaticity coordinates of (0.2092, 0.7167) was obtained by changing the thicknesses of NPB and TCTA; this resulted in standard green light (0.21, 0.71).