Since the uncut chip thickness is comparable to the cutting edge radius, in fact, micro cutting is a negative rake angle cutting process. The minimum chip thickness has great effect on the cutting process. This paper presents an experimental investigation of specific cutting energy and surface quality based on the negative effective rake angle in micro turning. A new model is developed to calculate the negative effective rake angle in micro cutting. The effective rake angle is found to be more negative with the decreasing ratio of uncut chip thickness to cutting edge radius. The minimum chip thickness is calculated to be about 0.2–0.3 times of cutting edge radius based on the critical negative rake angle. The turn point of the nonlinear increase of specific cutting energy is observed to be at the critical negative rake angle. The minimum surface roughness also is found to be achieved near the critical negative rake angle.