The effect of tungsten (W) addition on the hardenability of low-carbon boron steels was investigated usingdilatometry, microstructural observations and secondary ion mass spectroscopy. The hardenability was discussed with respectto transformation behaviour aspects depending on the segregation and precipitation of boron at austenite grain boundaries. Acritical cooling rate producing a hardness corresponding to 90 % martensite structure was measured from a hardness distributionplot, and was used as a criterion to estimate hardenability at faster cooling rates. In the low-carbon boron steel, the additionof 0.50 wt.% W was comparable to that of 0.20 wt.% molybdenum in terms of critical cooling rate, indicating hardenabilityat faster cooling rates. However, the addition of 0.50 wt.% W was not more effective than the addition of . 0.20 wt.%molybdenum at slower cooling rates. The addition of 0.20 wt.% molybdenum completely suppressed the formation of eutectoidferrite even at the slow cooling rate of 0.2 oC/s, while the addition of 0.50 wt.% W did not, even at the cooling rate of 1.0oC/s. Therefore, it was found that the effect of alloying elements on the hardenability of low-carbon boron steels can bedifferently evaluated according to cooling rate.