Combined effects of thickness and temperature on essential work of fracture (EWF) of polybutylene terephthalate (PBT) film were studied using single edge notched tension (SENT) and double edge notched tension specimens. It is found that specific essential work of fracture (we) for PBT is independent of temperature below T-g (approximate to 80 degreesC). but decreases above T-g. Between temperatures 25 and 100 degreesC, w(e), was independent of film thickness in the range 0.125-0.375 mm. The specific non-essential work of fracture (betaw(p)) was temperature and thickness dependent, being greater for the SENT type specimens. Specimen orientation had no influence on w(e) but strongly affected betaw(p). It was found that betaw(p) is greater for cracks propagating normal to the extrusion direction as compared to the parallel direction.