The electrical and the optical properties of tandem organic light-emitting devices (OLEDs) with stacked electroluminescence units were investigated to clarify the charge-generation mechanisms due to the existence of a charge-generation layer (CGL). The current density of the current limited devices with an 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) CGL was 35% higher than that of devices with a tungsten-oxide (WO3) CGL. The maximum current density of the current limited devices with a HAT-CN CGL was as high as 259 mA/cm2. The brightness of the tandem OLEDs with a HAT-CN CGL was 15% higher than that of the tandem OLEDs with a WO3 CGL due to an increase in the current density. The charge-generation mechanisms of tandem OLEDs with a CGL were described on the basis of the experimental results.