The electrolytic boronizing on carbon steels in the mixture of $Na_2B_4O_7$ and NaCl was conducted at 750~$950^{circ}C$ for 1~6 hours under 0.5A/$ extrm{cm}^2$ current density. The micorostructrure and microheredness of boronized layer was also studien. The effect of the additive such as $CaCl_2$ or NaOH on the formation of boronized layer was also investigated. The boronized layer were composed of two sublayers, i.e., FeB and $Fe_2B$ , which have tooth structure. the average layer thinknesses of the low carbon steel and SM45C boronized at $900^{circ}C$ for 4hours were 153 and 138 $mu extrm{m}$, respectively. The thickness of the twosublayers was significantly increased with increasing boronizing temperature. To obtain a single $Fe_2B$ layer without FeB sublayer, the boronized materials ware homegenized at $950^{circ}C$ for 4 hours. It was fount that the single layer with a microhardness Hv$ hickapprox$ 1120 -1250 was formed. The calculated activation energies for formation of boronized layer on the low carbon steel and SM45C were 18.7 and 12.6 Kcal/mol, respectively.