One of the promising methods of increasing surface hardness of engineering tools is boronizing. The boronized parts have extreme hardness exceeding 2000 HV, excellent mechanical properties and corrosion resistance. 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 the temperature of 850° and 950°C for 2, 4, 6 and 8 hours. Boride layers were examined by optical microscope (OM), scanning electron microscope (SEM) and X-ray diffraction (XRD). Mechanical properties of boride layer were characterized with hardness and fracture toughness measurement. Boride layer hardness was measured by knoop indenter under load of 0,5N and fracture toughness of borided surfaces for 950°C was measured using Vickers indenters with a load of 4N. Metallographic and XRD analysis revealed that single-type Fe2B layers were formed on the surface of EN-C35E steel. Depending on boronizing time and temperature, it was found that the hardness of boride layer ranged from 1895-2143 HK 0.05 that is nearly 8 times higher than the steel substrate hardness. It was observed that the fracture toughness of boride layer ranged from 3.60 to 4.20 MPa m1/2.