The coupling effect of thermal and mechanical behaviour of metallic materials was studied by uniaxial tensile test. Heat transfer exists as the specimen is loaded from zero over elastic to plastic region. Cooling occurs in the elastic region and heating in the plastic region. Both theoretical model and experimental results are presented. The theoretically derived model, Θ = − (αT o/ρC v)σ zz , which takes exclusively thermoelastic effect into account, explains the cooling phenomenon well. The thermoelastic limit coincides with yield where the temperature drops to its lowest value. This is characterized by the occurrence of an inversion of the trend of temperature against stress. Experimental results on normalized AISI 1045 mild steel reveal a relative maximum temperature drop of about 0.4 K, which is slightly less than as theoretically predicted. Instead of the usual 0.2% offset method, the lowest temperature method introduced here will be preferred in defining the yield stress for its unambiguous and empirical evidence.