The aim of the present study was to investigate the residual stress field in a steel gear cut and machined from extruded bars and then submitted to a case hardening surface treatment. Triaxial neutron diffraction measurements were carried out for residual stress determination. The unstrained lattice parameters in the outer layer and the bulk were measured using two disks made of cemented and ferritic steel, respectively. The residual stress field was measured in different points, both in a tooth and in a throat between two teeth. The results show residual stress profiles with compressive values (similar to400 MPa) just below the surface and tensile ones (similar to200 MPa) in the bulk. The inversion point has been found to lie exactly were the hardness of the material reaches its bulk value. It is observed that the martensitic transformation causes a volume increase that is comparable to that brought about by nitriding treatments. Thus residual stress is also comparable for the two hardening techniques.