The property such as microhardness of boride layer formed on St37 iron was investigated. Boronizing was carried out in a solid medium consisting of nano size powders of 50% B4C as a donor, 45% SiC as a diluent, and 5% KBF4 as an activator treated at the temperature of 10000C for 8 hours. The phases that were formed on the substrate was found as Fe2B and FeB layer that had smooth and flate shape morphology. The hardness of boride layer on St37 was over 2000 HV, while the hardness of untreated St37 iron was about 123,82 HV. Depending on process time and temperature, the depth of boride layer ranges from 20 to 60 μm, leading to a diffusion controlled process.