Fracture toughness of a semi-crystalline polyethylene naphthalate (PEN) film of thicknesses 0.050, 0.075 and 0.125 mm was measured as a function of temperature and loading rate using both double edge notched tension (DENT) and single edge notched tension (SENT) specimens. The specific essential work of fracture (EWF) and the multi-specimen J-integral methods were used to evaluate fracture toughness. The variation of the specific total work of fracture (w(f)) with ligament length (L) was linear for ligament lengths between 5 and 15 mm. Within this range, wf versus L was independent of thickness at all temperatures but was dependent on both temperature and loading rate. The specific EWF (w(e)) was found to be independent of thickness and loading rate but showed three regions of varying temperature dependence. Between 23 and 80 degreesC (region I) w(e) was essentially independent of temperature but increased with temperature between 80 and 120 degreesC (region II) and decreased with temperature thereafter (region III). At glass transition temperature (i.e. 120 degreesC), w(e) reached a maximum value of 75 kJ/m(2). The specific non-EWF(betaw(p)) increased with both loading rate and temperature. The greatest change in betaw(p) value with respect to temperature was obtained in region II. The plot of J-integral versus crack extension (Deltaa) was independent of thickness but was dependent upon temperature. we was found to be equivalent to both J(0.2) and J(0).