An in-situ microfibrillar blend based on poly(ethylene terephthalate) (PET) and polyethylene (PE) was fabricated through slit die extrusion, hot stretching, and quenching. The morphological characteristics of the PET phase, such as diameter and its distribution, which were observed after the matrix was etched away, appear to be dependent on the hot stretch ratio at a fixed blend composition. The increase of the hot stretching ratio makes the PET particles change from spheres and ellipsoids to rodlike particles, and finally to well-defined microfibers. The fracture toughness of the in-situ microfibrillar blend was evaluated using deeply double-edge notched tension (DDENT) specimens according to the essential work of fracture procedure. Initially, the increase of hot stretch ratio makes the specific essential work of fracture (w(e)) rise. A maximum w(e) appears at 25.4. Further increase of hot stretch ratio causes a slight decrease of w(e). On the other hand, it shows that lower hot stretch ratios make the specific non-essential work of fracture (w(p)) rise slightly. As it exceeds 6.4, w(p) decreases substantially. It was believed that the characteristics of the PET domains were responsible for the fracture behaviors of the in-situ microfibrillar blend. (C) 2004 Society of Plastics Engineers.