The effect of molecular weight between crosslinks, (M) over bar(c), on the fracture toughness and fracture behaviour of diallylterephthalate resin was investigated. As (M) over bar(c) increased, the fracture toughness increased rapidly up to a maximum value and then decreased gradually. Scanning electron micrographs showed that the fracture toughness increased with an increase in the length of the slow crack growth, i.e. plastic deformation zone. Also, fracture toughness was found to have a close relation to crack opening displacement. Such a correlation reveals that the difference in fracture toughness with (M) over bar(c) was due to the different sizes of the plastic zone. It is suggested that diallylterephthalate resin could undergo deformation via shear yielding at the crack tip with subsequent crack blunting. There was no evidence that crazes occurred in the fracture process.