The electrospinning technique was used to prepare the Polyethylene Terephthalate (PET) mats. The electrospun PET mats with different fiber diameters have many applications such as biotechnological and biomedical, functional coating, lithum-ion membrane and filtering membrane and so on. Several techniques including scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and single fiber electronic tensile strength tester were used to characterize the electrospun PET mats before/after thermal treatment. When choosing the suitable temperature, the thermal treatment could improve the tensile properties without damage the structure of electrospun PET nanofiber mat, if the suitable temperature was been choosed.