The following theoretical equation has been obtained for measuring the rate of slow crack growth in polyethylene in terms of the crack opening displacement rate δ: δ ̇ = α y(1−y 2) 2 ηd oE 2α 2 c K 4 Here δ y is the yield point, K is the stress intensity, η is the intrinsic viscosity of the fibrils in the craze, E is Young's modulus, δ c is the stress to produce a craze, d 0 is the primordial thickness from which the craze originates and γ is Poisson's ratio. The theoretical equation agrees with the experimental observation: δCK 4 e- −Q/RT Thus, for the first time, the dependence of δ on stress and notch depth have been derived in fundamental terms and the physical parameters that constitute the factor C have been identified. The intrinsic viscosity η can be calculated from the theory using specific experimental data. For example at 42°C, the fibrils in a craze in a homopolymer have an intrinsic viscosity of 3 × 10 11 Pas. This is much larger than the melt viscosity of the amorphous region, which is about 10 5–10 6 Pas. Thus, the resistance of polyethylene to slow crack growth is governed by the crystals and not by the amorphous region.