Abstract: The phenomenon of ductile-brittle transition and the measurement of dynamic fracture toughness of metallic materials under impact loading are important parts of the research on dynamic mechanical properties of metal materials. In view of the lack of understanding of ductile-brittle transition of metallic materials under impact loading and the difficulty in measuring the dynamic J-resistance curve of ductile materials at relative low loading rate, a method is proposed to measure the ductile-brittle transition process of 15MnTi and 11MnNiMo steels at different loading rates, and the effect of crack tip constraint on the rate change of dynamic ductile-brittle transition of the two materials, by using high-speed material testing machine and its corresponding special fixtures. The dynamic fracture toughness of 15MnTi steel under low loading rate was measured by adjusting the length of compression bar and changing the crack propagation by means of the brake of upper roller. The experimental results indicate that the CT specimen of 15MnTi steel is characterized as ductile fracture when loading velocity is lower than 0.025~m/s, and the fracture character of CT specimen of 15MnTi steel is ductile-brittle combination when loading velocity is between 0.1~m/s and 0.5~m/s, and brittle fracture of the CT specimen of 15MnTi steel starts from 0.5~m/s. The phenomenon of brittle fracture followed by ductile fracture for the CT specimen of 11MnNiMo steel occurs when the loading rate is greater than 1.5~m/s. The dynamic brittle fracture rate of 15MnTi and 11MnNiMo steels is significantly affected by crack tip constraint, and the dynamic brittle fracture rate of the material decreases obviously with the increase of in-plane constraint and out of plane constraint. It is also found that in the three-point bending tests, the fracture toughness of 15MnTi steel decreases slowly with the increase of loading rate, when the loading rate is lower than 8788~MPa\cdotmm/s.