Tribological properties of nitrided AISI 4340 steel are evaluated using ball-on-disk tribometer in vacuum. Optical microscopy and Vickers hardness tester are used to analyse compound layer thickness and case depth. X-ray diffraction is used to observe the presence of different constituents of nitrided surface before and after the wear tests. The wear mechanisms are investigated by microscopic observations of the worn surface using scanning electron microscopy. At 200 rpm, mild adhesive wear forms the main wear mechanism under the lower load. With increasing the normal load, the dominant wear mechanism changes to mild plastic deformation and abrasive wear. At 600 rpm, the wear mechanisms vary from the mild plastic deformation to severe plastic deformation, abrasive wear and even delamination with increasing the normal load.