The electrical and optical properties of tandem organic light-emitting diodes (OLEDs), in which a fluorescent and phosphorescent emitting units are connected by an organic charge-generation layer (CGL), were experimentally analyzed. To investigate the internal properties of the tandem OLEDs, we fabricated and compared two single, two homo-tandem, and two hetero-tandem OLEDs using the fluorescent and phosphorescent units. From the experimental results of the OLEDs obtained at the same current density, the voltage across the CGL as well as the individual emission spectra and luminance of each unit of tandem OLEDs were obtained and compared with the theoretical simulation results. The analysis method proposed in this study can be utilized as a method to verify the accuracy of optical or electrical computer simulation of tandem OLED and it will be useful to understand the overall electrical and optical characteristics of tandem OLEDs.