Diamond-like carbon (DLC) films are effectively utilized to improve the tribological properties of materials due to their low friction coefficient, good wear resistance and high hardness. In recent years, different pre-treatments such as plasma assisted diffusional processes have been used to increase long-term durability of these coatings. In the present work, DLC films were deposited on untreated and plasma nitrided AISI 4140 low-alloy steel samples using physical vapor deposition method (PVD). The effect of nitride layers on the structural and mechanical properties of DLC films was examined by XRD, SEM, microhardness tester and reciprocating tribotester. The wear tests were performed under dry, saltwater and lubricant environments. The microhardness results indicated that surface hardness considerably increased after duplex treatment and this caused to increase plastic deformation resistance of the material. The reciprocating test results showed that DLC films deposited on untreated and plasma nitrided samples reduced the coefficient of friction because of the formation of transfer film between sliding/mating surfaces. Also, wear rates improved significantly after duplex treatment in consequence of high plastic deformation resistance of duplex treated samples. The highest wear resistance was obtained in lubricated condition while the lowest wear resistance was for the samples tested in dry conditions.