The phase transformation behavior of 9Cr–3W–3Co–VNb steels with boron contents of 47 and 130 ppm has been investigated using differential thermal analysis (DTA). In our DTA experiments, disk-shaped samples were normalized and then tempered at temperatures between 10 and −50 °C from the ferrite-to-austenite transformation temperature (Ac1) at a rate of 30 °C/min. The measured Ac1 temperatures for 47 ppm B steel and 130 ppm B steel were 878 and 884 °C, respectively. The general features of the phase transformation behavior and the changes in hardness with tempering temperature for these boron containing steels were almost identical, irrespective of the boron content. In the DTA cooling curves during tempering at temperatures between −30 and 10 °C from the Ac1 temperatures, an exothermic peak due to the formation of fresh martensite was observed and the peak area increased with increasing tempering temperature. The Vickers hardness values measured after the DTA experiments decreased gradually with increasing tempering temperature, reaching a minimum value of about 180 HV around Ac1−40 °C, and then increased markedly, reaching a value of about 350 HV at Ac1+10 °C.