Hot stamping of quenchable boron steels is a new and complex forming technology integrating metal hot forming and quenching process. In order to gain the Time – Temperature - Transformation (TTT) curves of 22MnB5, the phase-transformation temperatures of boron steel 22MnB5 are measured by DIL 805A/D quenching and deformation dilatometer. According to the expansion curves of phase-transformation at some constant temperatures, the start and finish temperatures of phase-transformation are calculated by using the expansion method and the tangential method. The types of metallographic phases after phase-transformation are judged by the optical microscope and micro hardness. The TTT curves of 22MnB5 are drawn in terms of the results of testing and calculating.