Boronizing, which involves diffusion of boron atoms into steel substrate to form hard iron borides is well known diffusion coating technique. In this study, salt bath boronizing processes were performed on EN-C35E steel substrate in slurry salt bath containing borax, boric acid as boron sources and ferro-silicon as reductant. The process was performed at 850 and 950degreesC for 2, 4, 6 and 8 hours. Boride layers were examined by optical microscope (OM), scanning electron microscope (SEM) and X-ray diffraction (XRD). Hardness of borides formed on the steel substrate was measured by Knoop indenter under load of 0.5N. Metallographic studies and XRD analysis revealed that single-type Fe2B layers were formed. Depending on boronizing time and temperature, it has found that the hardness of boride layer ranged from 1895-2143 HK0.05 that is nearly 8 times higher than substrate hardness. The thickness of the layer ranged from 25 to 167 mum depending on boronizing time and temperature.