We conducted an investigation of the stability of organic electroluminescent (EL) diode during operation and storage using SEM, XPS, cyclic voltammetry, etc. We found that one must pay attention to the following points; 1) morphological stability of organic thin films, 2) electrochemical stability of organic materials themselves, 3) oxidation and/or peeling-off of the cathode, 4) fabrication process. Uniformity of the organic layers made from molecular materials is essentially important to attain a long-term stability of the EL diode. It improved the contact with the cathode resulting in suppression of dark spots and stable operation.