AbstractAn industrial advantage of laser processing is that it permits local treatment of mechanical components on those specific areas subject to high stress levels. The way in which this type of laser treatment enhances structural steel properties has been studied. The 42CD4 and X C42 steel specimens were notched to create a stress concentrationfactor identical to that observedfor gears. The work involved optimising three parameters: the laser beam itself, i.e. the beam energy-time of interaction couple; specimen preparation, i.e. selection of the best laser beam-specimen coupling medium (phosphating gave the best emissivity coefficient while avoiding specimen contamination); the metallurgical heat treatment, designed to obtain both high toughness and good ductility. Specimens tempered at various temperatures before the laser treatment were fatigue tested. The results enabled the selection of an intermediate tempering temperature resulting in the highest fatigue limit. They also showed that, in regard to fatigue behaviour, laser surface treatment compared favourably with thermochemical carbonitriding.