The effect of heat treatment to the polymer hole transport layer (HTL) on the performance of the organic light emitting devices (OLEDs) was investigated. Poly-N-(p-diphenylamine) phenyl methacrylamide [PDPMA] was used as a polymer HTL. Polymer HTL was coated onto ITO substrate by spin coating method. The effect of polymer HTL degradation on the turn-on voltage of the prepared OLEDs was investigated by I-V measurement at the various conditions of temperature and exposure time. OLEDs were fabricated with the structure of ITO/Polymer HTL/Emitting layer (Alq(3))/Electrode (Al), and polymer HTL was treated by heat. The different surface morphologies of polymer HTL was observed by atomic force microscopy (AFM) at the variation of exposure time and temperature. The degree of the polymer HTL degradation was increased as the exposure time and temperature were increased, and it was observed that the increase of turn-on voltage was primarily caused by the polymer HTL degradation.