To reduce the fuel consumption as well as to improve the crash safety of vehicles, the usage of hot stamping parts is increasing dramatically in recent years. Aisin Takaoka has produced hot stamping parts since 2001 and has developed various technologies related to Hot Stamping. In an actual hot stamping process, parts with insufficient strength could be produced sometimes at a prototyping phase, even under the proper forming conditions. In order to understand these phenomena, in this paper, phase transformation in a boron steel 22MnB5 under various cooling rates were investigated and the effects of pre-strain conditions on the phase transformations were characterised. Uniaxial tensile specimens were stretched under isothermal conditions to different strain levels of 0-0.3, at strain rates of 0.1-5.0/s and deformation temperatures of 650-800°C. A dilatometer was used to measure the dimensional changes of the specimen to rationalize the phase transformation of the boron steel during rapid cooling after deformation. The results showed that the deformation at austenite phase causes the reduction in martensitic phase transformation. For a certain strain level, deformation applied at a lower temperature and a higher strain rate would result in less amount of martensitic phase transformation. In addition, the relations between hot stamping conditions and Vickers hardness, which are normally used as a daily quality check in the industry, are also presented in the paper.