Abstract: Experiments of high speed cutting were carried out on the hardened steel 4Cr5MoSiV1.The evolution of chip morphology changed with the cutting speed and the microstructural characteristics in adiabatic shear bands (ASB) were observed by optical microscopy and scanning electron microscopy (SEM).The main factors affecting forming and developing of ASB and the distribution of ASB under various cutting conditions were analyzed.The results indicate that the cutting speed was one of the main factors affecting forming and developing of ASB,and the length and the width of ASB reduced gradually with the increase of cutting speed,the distance between two ASBs increased gradually.The rake angle of cutting tool was the secondary factor affecting the forming and developing of ASB.