In micro-machining operations, a small amount of material is removed and very small cutting forces are created. Micro-end mills with less than 1-mm diameter have a short and unpredictable tool life. When these tools fail their shafts are broken. An operator cannot detect tool breakage from the sound or by a visual inspection during the machining operation. In this paper, the tool-breakage mechanism of micro-end mills, wear-induced stress (WIS), is explained by evaluating the thrust direction cutting force characteristics, by inspecting scanning electron microscope (SEM) images of tool tips and acoustic emission activity.