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In this study, the effect of Na 2 S 2 O 3 on RE-boronizing behavior of 45 carbon steel was investigated by varying the Na 2 S 2 O 3 content in the treatment agent. The content of Na 2 S 2 O 3 was set to 0%, 1%, 2%, 3%, 4% and 5%, respectively, to study the effect of sulfur on the property of the boride layer. The diffusion layers obtained by the above-described agents containing different levels of Na 2 S 2 O 3 were named B-0SL, B-1SL, B-2SL, B-3SL, B-4SL and B-5SL, respectively. Pack RE-boronizing of 45 carbon steel was carried out at 300 °C for 2 h, and then at 850 °C for 6 h. The diffusion coatings were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Vickers microhardness measurement, profilometry, the Daimler-Benz Rockwell-C adhesion test, and friction test. Testing results have revealed a significant morphology change in the borides with different Na 2 S 2 O 3 contents. The thickness of the layer increases dramatically with increasing content of Na 2 S 2 O 3 , reaches its maximum (152 μm) with an agent of 3 wt.% Na 2 S 2 O 3 , then remains unchanged with further increasing Na 2 S 2 O 3 content. The concentrations of B, S and O in the diffusion coatings increase with increasing the content of Na 2 S 2 O 3 . B-rich, O-rich and S-rich regions form successively from the surface to the inner layer in the B-5SL layer. FeS 2 and Fe 2 O 3 phases with a low hardness were observed in the B-3SL and B-5SL layers, respectively. The microhardness of the B-5SL layer is the highest, while the adhesion between the coating and the substrate in the B-3SL layer is the best among all the coatings. Compared with other coatings, the B-3SL layer can still keep a lower friction coefficient as well as a lower wear rate even when a high load is applied. [ABSTRACT FROM AUTHOR]