Orthogonal cutting experiments of hardness AISI1045 steel( HRC45) are performed. The change of chip formation and cutting force with cutting conditions are investigated through metallurgical observation, and the critical cutting speed of adiabatic shear of AISI1045 steel at different cutting depths and rake angles are given. In addition, the saw-frequency and space length of serrated chip and the influence of serrated chip on cutting force are also studied. The critical cutting speed decreases with the increase of cutting depth, but increases with the increase of rake angle. The saw-frequency of serrated chip is found to be independent of the cutting depth, and increases with the increase of cutting speed and rake angle. The cutting force decreases with the increase of cutting speed and rake angle, but increases with the increase of cutting depth, and there isn’t a sudden change of cutting force at the onset of adiabatic shear.