Duplex plasma surface treatments of chromizing and plasma nitriding on various alloys such as mild steel (AISI 1020), AISI H13, and 1Cr–0.5Mo steel (ASTM A213) were carried out. Specimens were chromized at 1200–1300°C for 5 h and were subsequently plasma nitrided at 530°C for 1 h. Effects of plasma nitriding on the Cr diffusion coating and the characteristics of the final duplex-treated specimens were analyzed using SEM, EDS, XRD, and microhardness tester. The chromizing for 5 h by pack cementation process had created a Cr diffusion layer of approximately 150–300 μm thickness. Subsequent plasma nitriding on the Cr-diffused layer induced formation of a duplex-treated surface layer with a largely improved microhardness up to approximately 1500 Hv (50 gf). The main cause of the large improvement in surface hardness was due to the fact that CrN and FexN phases were created by chromizing and plasma nitriding treatment. Also being studied was the high temperature wear resistance of the duplex-treated specimens at 600°C. Comparing the duplex-treated specimens with the specimens treated only by chromizing, the wear volume of the duplex-treated AISI 1020 and H13 steels after a wear test at 600°C were reduced by a factor of 8 and 3, respectively. Also the examination of the wear tracks after wear test at 600°C on chromized and duplex-treated specimens indicated that regardless of the alloy compositions, the wear tracks of the duplex-treated specimen showed an abrasive type wear while the chromized specimens showed an adhesive type wear.