In the electroluminescent (EL) device, the joule heat, which is caused by non-emissive site, is the crucial factors mainly affecting the degradation process of the organic electroluminescent device (OELDs). In this thesis, the effect of heat treatment to the polymer hole transport layer (PHTL) on the performance of the organic electroluminescent devices was investigated. Poly(9-vinylcarbazole) [PVK] was used as a PHTL. PHTL was coated onto ITO-coated glass substrate by spin coating method. OELDs were fabricated with the structure of ITO/PHTL/Emitting layer (Alq3)/ Electrode (Al), and PHTL were treated by heat. The different surface morphologies of PHTL were observed by atomic force microscopy (AFM) at the variation of exposure time and temperature. The degree of the PHTL degradation was increased as the exposure time and temperature were increased.