Abstract In this study, the effects of boronizing treatment on material's dimensional changes and surface roughness were investigated. The parameters chosen were substrate material composition, surface roughness before boronizing treatment, and boronizing time. The AISI 1020, AISI 1040 and AISI 2714 were chosen as substrate materials whereas Ekabor I was selected as boronizing powder. Materials were boronized at 900 °C by using a solid boronizing method for 2 or 4 h. The gradual growth of boride layer on the surface, dimensional changes and their effects on surface roughness were investigated. Variations in topographical surface roughness were determined by SEM. With the boronizing treatment, dimensional increases of the material's were observed. The dimensional increase was one fifth of boride layer thickness for AISI 1020 or AISI 1040, whereas it was one third of boride layer thickness for AISI 2714. Boronizing treatment had also a significant effect on surface roughness of materials. A “threshold roughness” term was defined in our study. This term is a surface roughness value for smooth surfaces, which received boronizing treatment. For the same material and with the same boronizing conditions, the threshold roughness value was achieved after boronizing, when surface roughness of material was below the threshold roughness value, before boronizing was applied. However, when surface roughness of a material was above that threshold roughness value, surface roughness value decreased with boronizing treatment. The thereshold roughness value depended on substrate material composition and boronizing parameters.