At present, the direct experimental observation and confirmation of thermal damage on semiconductors remain unexplored. This report presents clear evidence of the thermal irradiation damage on the electrode contact region of an organic semiconductor in the electrode deposition process by applying an ultralong uniform rubrene single crystal to simultaneously fabricate vacuum-deposited electrode and stamped electrode top-contact organic field-effect transistors (OFETs). The surface potential measurements combined with the transfer characteristics show that the thermal irradiation damage causes the HOMO level of rubrene shift downwards by 0.17 eV, resulting in the enhanced contact resistance and hence the lowered field-effect mobility. This interesting experimental discovery clarifies the nature of vacuum-deposited electrode induced thermal irradiation damage on OFETs, will enhance the understanding and development of contact engineering to further improve the performance of organic electronics.