A theory is developed to describe the fracture probability of high impact polystyrene. By assuming that fracture initiation is a consequence of the impingement of crazes due to the craze thickening process, an equation for the fracture probability as a function of the strain is derived. Experiments support the validity of this equation qualitatively while the stress and temperature dependence of the fracture probability correlate in the expected manner with the strain rate. An approximation allows the calculation of the fracture probability as a function of time.