Essential work of fracture (EWF) analysis was used to study the effect of loading rate (v = 2, 5, 25 and 50 mm/min) on fracture toughness of PET film of thicknesses 0.175, 0.275, 0.375 and 0.5 mm. Using single edge notched tension (SENT) specimens, it was found that the specific essential work of fracture, w(e), increases slightly with increasing loading rate but Within the specified range of loading rate, it showed no significant variation with respect to thickness. The specific non-essential work of fracture, beta w(p), decreased with increasing rate and increasing thickness.