The boronizing effects of STD 61 steel have been studied on the micro structure and hardness. The STD 61 Steel was soaked in molten salt, consisted of KCl, $BaCl_2$, NaF, $B_2O_3$, FeB, and Ce, at various temperatures and times. The boronizing conditions for the peak hardness were the temperature range of $900^{circ}C$ to $950^{circ}C$ for 5 hr and that of $1000^{circ}C$ for 3 hr, respectively. Four boride layers such as FeB, $Fe_2B$, ${alpha}$ and matrix layer surface were observed from the microscopic surface examination. The thickness of boride layer was increased by increasing the boronizing time and the temperature. The structure of boride layer was tooth shape.