The influence of trace boron and cooling rate on the thermal embrittlement of 2.25Cr -1Mo steel was investigated by thermo-mechanical simulator and SEM analysis. It was proved that the trough in the thermo-ductility curve located between 1000 degrees C and 700 degrees C and shifted to lower temperature and the temperature range of the trough became shallow and narrow as the boron content increased. When the cooling rate decreased, the ductility trough became shallow and narrow too. This made the steel free of surface crack in CC process.