A model of chip formation for high-speed machining was proposed. A serial of parameters were built to assess the deformation of the chip segment. Turning experiments were carried out on a kind of high-strength steel, AerMet100, to validate the model. A further discussions were made on the experimental results on which a model of chip formation at different cutting speeds was developed. The mechanisms of chip deformation at the different cutting speeds were studied qualitatively. It was found that with the increase of cutting speed, the material dynamic yield strength, tensile strength would be strengthened and the influence of thermal softening effect, strain hardening effect would be weakened, which could decidedly lead to the onset of serrated chip.