Abstract: This work deals with the experimental and analytical investigations of the formation mechanisms and the plastic flow stability of chips in the orthogonal cutting processes. In the cutting experiments, four kinds of metal are chosen as modelling materials. In the high-speed cutting process of each testing metal, we observed the transformation of chip morphology from continuous to serrated and determined the critical cutting speed which depends on the material properties of workpiece and the cutting conditions. Based on the experimental results, a two-dimensional orthogonal cutting model is proposed for analyzing the two-dimensional effects of chip flow and a corresponding basic theoretical framework is established under the plane strain loading condition. By introducing a group of scaling quantities that related to the cutting condition parameters, a system of dimensionless governing equations is obtained by normalization, a main dimensionless controlling parameter is determined in terms of experimental conditions and numerical simulation results, an instability criterion is established by the linear perturbation analysis under plane strain loading conditions, and the asymptotic flow fields on the velocity and stress in the extended chip formation zone are obtained by approximate analysis of chip flow. The theoretical results showed that, provided the cutting speed is sufficient high, the plastic flow of continuous chip will be unstable. This instability behavior of chip would be the non-localization unstable flow of the continuous chip caused by plane loadings and differs from the shear-localized instability in the serrated chip. The dimensionless controlling parameter, called as the modified Reynold number, did play a leading role since it better describes the plastic instability behavior of continuous chip flow and the shear-localized instability behavior of the serrated chip in the orthogonal cutting process of metals.