Hardenability and mechanical properties of boron-bearing steels containing C, Mo and Cr were investigated in this study. Using quench dilatometer, the steel specimens were cooled down to room temperature at different cooling rates to construct continuous cooling transformation diagrams and then the transformation products from austenite were examined. A critical cooling rate was introduced as an index to quantitatively evaluate the hardenability. The C addition to boron-bearing steels did not significantly affect hardenability compared to boron-free steels although it increases the hardenability. With the same content, the Mo addition largely increased the hardenability of boron-bearing steels than the Cr addition because it decreased both the transformation start and finish temperatures at low cooling rates. In particular, the Mo addition completely suppressed the formation of eutectoid ferrite even at the slow cooling rate of $0.2^{circ}C/s$, whereas the Cr addition nearly suppressed it at the cooling rates above $3^{circ}C/s$.