The thermal stability of organic EL devices was investigated by using an infrared thermal imaging radiometer, SEM, AFM, and fluorescence spectroscopy. The morphology of the electroluminescent (EL) devices was changed by high current flow which led to the increase in temperature of the devices. The interdiffusion between two organic layers was accelerated with the increase in temperature. The stability of the organic layers was improved by co-deposition of TPD-Alq as the hole transport layer.