An optical microcavity is a structure with at least one dimension on the order of an optical wavelength. The interaction between material and light within microcavity was changed greatly compared with the condition of free space. Cavity effects such as spectral narrowing and intensity enhancement of spontaneous emission have been observed. These effects can be used to improve the electroluminesence (EL) performances of organic light emitting device (OLED). The microcavity structure of green light OLED was optimized. The device was fabricated and studied with a structure of Glass/DBR/ITO/NPB/Alq:Rubrene/Alq/MgAg. The maximum luminance of 40100 cd/m2, maximum luminous efficiency of 6.44 cd/A, and a full width at half maximum (FWHM )of 28nm were obtained in microcavity OLED, whereas the maximum luminance of a noncavity device is 22580 cd/m2, maximum luminous efficiency2.98 cd/A, and FWHM120 nm. The EL peak intensity of the microcavity device is 4.2 times stronger than that of noncavity device of the same current density. It shows that introduction of microcavity into OLED improves not only the color purity, but also the luminous efficiency and luminance of the device.