Based on the thermodynamic model between TiN-BN-AlN in austenite, the effect of the boron content of direct-quenched low-carbon type Ti-B containing steels on the tensile strength and impact toughness was investigated. Variation of tensile strength showed a good relationship with the calculated soluble boron content at the start cooling temperature, showing that the thermodynamic model was applicable to the prediction of the hardenability of direct-quenched Ti-B containing steels. Impact toughness was varied form 20J to 250J at -40℃, depending on microstructure which was determined by the soluble boron content and the cooling rate. The highest absorbed energy of 200∼250J was obtained at martensite and lower-bainite mixed microstructure, followed by 100∼200J at ferrite and bainite mixed microstructure and 20∼40J at bainite with martensite-austenite constituent microstructure.