The fracture behavior of PET films of thicknesses 0.125, 0.175, 0.275, 0.375 and 0.5 mm was investigated according to Essential Work of Fracture (EWF) method. Single edge and double edge notched specimens of varying ligament lengths were tested in tension producing load-displacement traces that were typical of ductile failure. A linear relationship was obtained between the total specific work of fracture (w(f)) and ligament length (L). This linearity was maintained as ligament length exceeded the plastic zone size or one third of sample width. It was found that while the specific essential work of fracture (w(e)) was independent of thickness, the specific non-essential work of fracture (beta w(p)) decreased with increasing thickness. It was found also, that work of fracture parameters were independent of the specimen width for the range of thicknesses used in the present study. Nevertheless, for a sample width of 20 mm, a lower w(e) and a higher beta w(p) value was obtained. Good agreement was found between values of w(e) obtained from SENT and those obtained from DENT specimens; the value of beta w(p) was consistently higher for SENT specimens.