The fracture behaviour of two copolymers of polyethylene were studied under fatigue and constant load. There is an anomaly because one resin was stronger under fatigue loading and the other under a constant load. The fatigue parameters that were investigated were waveform, frequency, R, and time under maximum and minimum stress. The fatigue failure was described in terms of the stretching of the fibril in a craze under the maximum stress and bending of the fibril under the minimum stress, whereas only fibril stretching occurs under a constant load. The damage per cycle by stretching is much less than the damage per cycle by bending the fibril. It is proposed that molecular weight has a greater influence on fatigue fracture than on fracture under a constant load.