This work demonstrated the fabrication of white organic light-emitting devices (WOLEDs) using a dual emitting layer (d-EML) consisting of blue and ‘white’ emitters. In this d-EML system, the blue emitter not only emits but also assists the incomplete energy transfer. More importantly, it behaves as an effective trapping site for holes, which contributes to the efficient recombination of electron.hole pairs. The d-EML was constructed between the hole-transmitting layer (HTL) and the electron-transmitting layer (ETL) of Alq3 and BPhen. The thickness of the blue emitter used in the d-EML devices has an important effect on chromaticity and efficiency. Through the optimization of device structure, the reasonable white emission with Commission Internationale de L’Eclairage (CIE) color coordinates of (0.33, 0.33) and little color shift was obtained. The device showed an applicable luminance with its maximum luminance of 22,874 cd/m2 at a driving voltage of 16 V. The maximum luminance efficiency was achieved 8.10 cd/A, and the maximum power efficiency was reached 5.07 Lm/W. The result is explained with the help of the excitons generation and diffusion theory. According to the theory of excitons generation and diffusion, an equation has been set up which concerns electroluminescent spectra to the thickness of the two emitters and to the exciton diffusion length.