The material micro-hardness is used to reflect the plastic deformation within the localized shear band and the segment of saw-tooth chip. A high strength alloy steel Aermet 100 is taken as the workpiece material. The saw-tooth chip roots are obtained by cutting operation. The material micro-harnesses along the shortly initiated localized shear band, across the shortly initiated localized shear band and fully developed localized shear band, and within the segment are examined and analyzed. The localized shear band initiation and propagation are revealed by a new proposed model. The results show the localized shear band initiates firstly at the tool tip end of the primary shear band, and consequently another localized shear nucleates at the free surface end of the primary shear band. These two localized shears propagate face to face until the whole localized shear band is formed.