The temperature fluctuation during tensile testing of ABS (Acrylonitrile-Butadiene-Styrene), a typical thermoplastic copolymer with prefabricated defects, is experimentally studied in this paper. The initiation and evolution of the local temperature field near the defects are observed with infrared photography. It is shown that the specimen temperature decreases during the initial elastic deformation, whereas it rises in the following inelastic deformation process. According to the experiments, the heat generated from inelastic work is significant and approximates to 25-70 percent of the external work. Based on the microscopic and mesoscopic characteristics of deformation in glassy polymers, a preliminary and qualitative explanation of the cooling and heating phenomena is also presented.