The behavior of boride formation has been examined with solidification rates and boron contents by observing the solid/liquid interface using directional solidification in Mod. 12Cr-1Mo alloys. The phase transformation temperatures of liquidus, solidus, eutectic formation, and final solidification were analyzed by DTA. In order to analyze the crystallographic structure of the boride, extractive method, extracting the boride from the matrix, was used. The boride was proved as M3B2, and this boride phase expected to be formed by eutectic reaction. It has been also found that the boride formation depends on contents of born as well as the other alloying elements, such as C and W. As increasing boron contents, the amount of boride eutectic increased. Also, the alloying element W was shown to enhance the formation of the boride.